Cyanogenesis in Trifolium repens L. is under the control of two loci; Ac/ac and Li/Hi control cyanogenic glucoside and linamarase production respectively. Results obtained show that neither the dominant allele (Ac) coding for cyanogenic glucoside production nor the dominant allele (Li) coding for linamarase production are expressed in roots, seeds or seedlings before shoot emergence. Both linamarase and cyanogenic glucoside are produced during shoot growth and there is little turnover of cyanogenic glucoside in mature leaves. As the leaves senesce there is breakdown of the mechanism separating cyanogenic glucoside and linamarase, since cyanogenic glucoside is lost in plants of genotype Ac Ac Li Li but not in those of genotype Ac Ac Li Li. About 60% of the cyanogenic glucoside produced was lotaustralin, in shoots of plants which were fed with equal quantities of the precursor amino acids L-valine and L-isoleucine. In contrast, the proportion of cyanogenic glucoside as lotaustralin found in leaves of one plant, was only 40%. Different plants were shown to produce different quantities of cyanogenic glucoside, and the amount produced was dependent on temperature.