Arginine decarboxylase (ADC) is one of the key enzymes in the biosynthesis of putrescine in plants. The regulation of its activity depends on the physiological condition, developmental stage, and type of tissue. We have cloned ADC cDNA from soybean (Glycine max) hypocotyls to understand the regulation mechanisms of this enzyme activity. Using the cDNA clone, we examined the relationship between changes in the ADC activity and the level of ADC mRNA during development, in different tissues, and upon acid stress. The ADC activity began to increase 2 d after initiation of germination, reached a peak at the 5th d, and then declined. This change in the enzyme activity was preceded by similar changes in the level of the mRNA. The ADC activity was expressed tissue-specifically; this expression was well corelated with the mRNA content of the respective tissues. Incubation of the 5-d-old hypocotyls in pH 3 potassium phosphate solution caused a rapid increase in ADC activity. Within 2 h of acid treatment, the ADC activity increased more than threefold. This increase was also preceded by a corresponding increase in the mRNA content and was also regulated tissue-specifically. These results suggest that the change in the content of ADC mRNA has an important role in the regulation of the enzyme activity during early development, in different tissues, and upon acid stress.