1. The rat ventral prostate contains a heat stable inhibitor of Ca2+-dependent protease. This inhibitor was found to exist in a wide range of molecular weights (approx. 40-270 kDa) in adult rats. 2. However, in rats immediately post puberty (45 days of age) the inhibitor was predominantly of the higher molecular weight forms. 3. The inhibitor was also found in the dorsolateral and anterior (coagulating gland) prostate lobes but was of lower specific activity than in the ventral lobe. 4. Although the activities of the Ca2+-dependent protease and inhibitor decreased per ventral prostate gland after castration, these activities were not different during the first 10 days postcastration when expressed per g wet wt or per unit cytosol protein. 5. With a longer duration of castration, there was a decline in the specific activity (per unit protein) of the protease and an increase in that of the inhibitor. 6. Thus, the activities of the protease and inhibitor change in concert with the amount of cellular cytosol protein during the active period of castration-induced atrophy. 7. However, in long term castrated rats, functions carried out by the Ca2+-dependent protease may be effectively suppressed. 8. These data suggest that the Ca2+-activated protease probably is involved in the regulation of some metabolic processes in the active gland and is not prominent in the castration induced atrophy of the ventral prostate unless it functions through the proteolysis of some select protein(s).