High temperature delays abscisic acid (ABA) accumulation during water stress and can delay stomatal closure as well. This effect is difficult to study in a diurnally fluctuating environment. We stressed leaf discs of cotton (Gossypium hirsutum L.) by floating them on PEG 8000 solutions in a constant environment. Accumulation of ABA depended strongly upon temperature, displaying a maximum at 20 C even though plants were grown at much warmer glasshouse temperatures. At 35 , the amount accumulated was 45 to 80% less than at 20 . Leakage of ABA into the medium was not altered, by temperature. When leaf tissue was preloaded with (14)C-ABA, then incubated at 20 or 30 , the turnover (loss of (14)C) rate of ABA was increased by high temperature with a Q(10) of about 2. This enhanced turnover can account for the effects on ABA accumulation, assuming that synthesis is not similarly affected. Three radioactive metabolites of ABA were detected by TLC. Two of them had R(f)'s similar to phaseic and dihydrophaseic acids. The presumed dihydrophaseic acid accumulated without turnover at 20 , but was completely degraded at 35 . The third metabolite accumulated without apparent turnover at either temperature; it was hydrolyzed at alkaline pH to a compound not identifiable by R(f). The effect of temperature on ABA metabolism adds much complexity to diurnal patterns of ABA accumulation during episodes of water stress.