Puromycin aminonucleoside (PAN)-nephrotic rats have high serum angiotensin I-converting enzyme (ACE) activity. We studied ACE activity in serum, urine, and tissues from PAN-nephrotic rats on days 2, 6, 11, and 16 after PAN injection. Proteinuria and hypoproteinemia were evident on days 6 and 11. Though significantly decreased, proteinuria was still evident on day 16. Serum ACE activity increased on days 2, 6, and 11. Urinary ACE activity became evident on days 6, 11, and 16 and correlated positively with proteinuria, suggesting that the source of urine ACE is the blood serum. ACE activity increased in testis on days 2 and 6, in lungs and aorta on days 6 and 11, in adrenal glands and small intestine on day 11, and in kidney on days 11 and 16. Heart ACE activity decreased on days 2 and 6, and increased on day 16; brain ACE activity decreased on day 6 and increased on day 11. These data implicate that changes in tissue ACE content may contribute to elevate serum ACE in PAN-nephrotic rats.