Man-made chemicals pose a significant risk to ecosystems. However, with increasing awareness of environmental protection the last decades, acute lethal effects of chemical spills are rarely seen today. Successively, concerns about sublethal, chronic and long-term effects of low-concentration chemicals in the environment has grown. Sublethal effects are, however, more complex than dead/alive. Especially because organisms have various ways of establishing compensatory responses and avoid adverse toxic outcomes. The following work investigates some of the molecular dynamics that lead to or prevent toxicity and which act as sources of variation in hazard characterization of chemicals. Morespecifically, we aimed to study the effect of dose, timing and genetic variation on sublethal endpoints of aquatic organisms exposed to chemicals affecting endocrine functions. Initially a literature review was performed to explore one of the more unusual suspects of endocrine disruption: Nitrate. Through this review it became evident that studies were not easily comparable, for instance because of differences in experimental design with various species, age-groups, dosages and exposure durations being applied. This provided the basis for three experimental investigations into the dynamics of biological systems, which complicates the investigation of sublethal toxicity such as endocrine disruption. The small crustacean Daphnia magna, which is applied in standard tests of aquatic toxicology, was selected as model organism for these studies and the azole fungicide prochloraz, which has a wellcharacterized adverse outcome pathway (AOP), acted as model chemical. Initially the significance of genetic variation in model organisms was investigated in twolaboratory clones of D magna. The comparative study quantified life-history parameters under two different food sources, general cytochrome P450 (CYP) biotransformation capacity and related this to genetic variation in CYPs. Differences in life history parameters were found under the better feeding regime as one clone produced more but smaller neonates, while the other had fewer but larger. Different laboratory clones may therefore exhibit differentiating life history traits with possible consequences for the outcome of laboratory tests and hazard characterization based on these. The two clones also differed in baseline CYP-activity. Examination of single nucleotide variance of 47 CYP-regions in the two genomes did however not reveal a clear pattern that would explain the difference. This underlines the importance of good characterization of model organisms in studies of ecotoxicology Secondly, we turned to the question of timing. In a 3-generation setup with D. magna we investigated the transgenerational effect of prochloraz. Incorporating two exposure scenarios; one where all generations were exposed and one where only the first generation (F0) was exposed, we wished to study 1) acclimation and 2) effects beyond exposure. With the aim of adhering to the AOP framework, effects were studied at different levels of biological organization from transcriptomics to molecular measurements of CYP enzyme activity and key phenotypic effects, as growth and reproduction. For the continuously exposed animals, compensatory mechanisms and acclimation was seen over the three generations with changes in size and number of offspring as well as reproductive timing. CYP-activity was strongly inhibited in both F1 and F2 but gene transcription in F2 only showed small differences when compared to control animals and therefore seemed to have already acclimated to control levels. Effects of exposure in generation F0 seemed to skip a generation. In F2 decreased CYP enzyme activity and increased number of offspring was observed, and these endpoints were not affected in F1. Hence, grandmaternal effects seemed stronger than maternal effects. Such examples of effects beyond exposure need more investigation and their implications for the risk assessment of chemicals need to be elucidated. The study exemplified that the timing of exposure and effect assessment is a determining factor for the outcome of studies of sublethal endpoints. Finally, we investigated the questions of dose-dependency of toxic mode of action and tested the potential of using untargeted toxicometabolomics as a hypothesis-generating tool. A 48 hr test was performed with Daphnia magna (<24 hr old) at 5 sublethal concentrations of prochloraz. Subsequently, metabolites were extracted and analysed with liquid chromatography high resolution mass spectrometry (LC-HRMS) to investigate relative changes in the metabolome. Although a higher level of metabolite identification was not performed, a dose-dependent pattern was evident from a principal component analysis, with several metabolites following dose-response relationships. Likely switch points in the mode of action could also be identified. Untargeted toxicometabolomics therefore showed great potential as a hypothesis generating tool for identification of relevant key events in the adverse outcome pathways at the environmentally most relevant dose. Collectively these observations add to our knowledge of the dynamic response to sublethal toxicity and the molecular dynamics that lead to or prevent toxicity. However, further research is needed before we are able to incorporate these processes in testing of environmental risk of chemicals. A collective effort from the scientific community with resepect to data- and knowledge sharing is needed and maybe then we will be able to build predictive models that incorporates the dynamics and suits the complexity of biological systems