Changes in cystine aminopeptidase (CAP) activity in serum, placenta, uterus and liver in mice under various physiological conditions were examined by enzymatic analysis using S-benzyl-L-cysteine-4-methylcoumaryl-7-amide as a substrate. Changes in serum CAP activity during pregnancy were less remarkable than those reported in humans; the activity was highest at day 13 of pregnancy and lowest at day 17. During the estrous cycle, the serum CAP level was high at diestrus, as high as that at day 13 of pregnancy. The activity in the male serum was significantly low compared to that in the diestrous serum. Although the CAP level was especially high in the maternal placenta, levels were also high in the fetal placenta, uterus and liver as compared with the serum, suggesting CAP synthesis in these tissues. The CAP activity in the uterine tissue was lower in pregnant than normal cycling mice. Furthermore, the serum CAP activity was modulated by estrogen and progesterone both in females and males, and by androgen in males. The relevance of these findings to the physiological role of CAP was discussed.