Effects of abscisic acid(ABA) and temperature on the anthocyanin accumulation and phenylalanine ammonia lyase(PAL) activity were investigated in seedlings of Arabidopsis thaliana. In time course study, exogenous application of ABA (50-1000 μM) led to a noticeable increase in anthocyanin pigments which persisted over the following 5 days. Anthocyanins increased in concert with the chlorophyll loss. The activity of PAL, a key enzyme in the phenylpropanoid pathway, increased on exposure to ABA and reached maximum on the 4th day. This result shows that anthocyanin synthesis and PAL activity have a close physiological relationships. In the effects of temperatures (10℃, 17℃, 25℃ and 30℃) on anthocyanin accumulation and PAL activity in seedlings, a moderate-low temperatures (17℃) enhanced both anthocyanin content and PAL activity, whereas elevated temperatures (30℃) showed low levels of anthocyanin and PAL activity, suggesting a correlation between temperature-induced anthocyanin synthesis and the accumulation of PAL mRNA. Simultaneous application of ABA with temperatures induced higher anthocyanin synthesis and PAL activity in seedlings than ABA or temperature stress alone. Moderate-low temperature with ABA exposure elicited the maximal induction of anthocyanin synthesis and PAL activity. Therefore, ABA treatment significantly increased thermotolerance in A. thalinan seedlings. Ethephon and ABA showed similar mode of action in physiological effects on anthocyanin accumulation and PAL activity. Our data support that anthocyanins may be protective in preventing damage caused by environmental stresses and play an important role in the acquisition of freezing tolerance.