Leaves of the indoor ornamental plant Spathiphyllum wallisii have functional stomata only on the abaxial surface, yer irradiating the upper surface with UV-C (253.7 nm) leads to rapid closure of the stomata on the under surface, 5 or 6 cell layers away. Suspecting the intervention of chemical messengers, hormonal changes accompanying and following the 1 hour ultraviolet irradiation were studied. Within minues of such irradiation, ethylene released by the intact attached leaves more than quadrupled, but dropped to the control level within minutes of switching off the UV-C source. Sprays of the ethylene-releasing compound Ethrel had no effect on the stomatal resistance or the rate of transpiration. No evidence for a clear-cut role of ethylene in stomatal control could be obtained. The level of ABA dropped initially but then increased many-fold during the next 4 h, but during this period the stomata showed partial recovery. IAA level also decreased during the hour of irradiation, but increased dramatically up to 7 h. There was more IAA than ABA at all times and, since IAA alone promotes stomatal opening and partially overcomes the ABA-induced closure, an interplay of these two hormones is likely. Free phenolic acids extracted from UV-C irradiated leaves and freed of endogenous ABA, inhibited transpiration. Two of these compounds were putatively identified as p-OH-benzoic and proto-catechuic acids respectively, on the bases of their retention times in HPLC. The temporal sequence of hormonal and phenolic changes accompanying UV-C irradiation is: ethylene → ABA ≡ IAA → phenolic acids. Interaction among these compounds is suggested.