1. The release of kallikrein into the perfusate and urine of the isolated perfused rat kidney was studied.
2. Comparison between enzymic and immunological assays for kallikrein demonstrated the presence of an enzymically inactive form of kallikrein.
3. Of kallikrein found in normal rat urine 77 ± 4% is active and 23% is in an inactive form.
4. In the isolated perfused rat kidney a similar proportion of active kallikrein (84%) was excreted into the urine but very little enzymically active kallikrein (2%) could be detected in the perfusate.
5. However, significant amounts of enzymically inactive but immunologically reactive kallikrein could be found in the kidney perfusate. The rate of release of kallikrein into the perfusate was approximately one-fifth of the rate of release into the urine.
6. Renin showed a similar pattern of release into the perfusate and urine but the lysosomal enzyme marker acid phosphatase was not detectable.
7. These results show that kallikrein is secreted from the kidney into the circulation as well as being excreted in the urine. However, in urine the enzyme is predominantly in an enzymically active form whereas it is secreted into the circulation in an inactive form.