Iodine-labeled ([125I]) rat urinary kallikrein and rat urinary TAME esterase A2 were used as probes to look for urinary and plasma proteins that bind to these enzymes. Such proteins are presumptive enzyme inhibitors. Complexes formed with labeled enzymes were identified by polyacrylamide gel electrophoresis followed by autoradiography. Urine from young (6 weeks old) Dahl salt-sensitive (S) rats showed no, or only traces, of protein binding to kallikrein. Concomitant with the slow development of hypertension and proteinuria in S rats fed normal rat chow, one of the six kallikrein-binding proteins demonstrable in plasma was readily found in S-rat urine. This kallikrein-binding protein was called "KBP-1." R rats showed either no or much less KBP-1 in the urine, compared to S rats up to 5 months of age. A partly purified preparation of KBP-1 was shown to inhibit the TAME esterase activity of rat urinary kallikrein in the radiometric TAME assay. Urine of proteinuric S rats also contained two TAME esterase-binding proteins, TEBP-1 and TEBP-2, detected with the [125I]-esterase A2 probe. As S rats aged from 3 to 8 months, free KBP-1 disappeared from the urine in spite of increased and marked proteinuria and the continued presence of KBP-1 in plasma. Concomitant with this age-related loss of urinary KBP-1 there was a marked shift in S urinary proteins binding to [125I]-esterase A2 from TEBP-1 to TEBP-2. It was speculated that KBP-1 and TEBP-1 were the same protein detectable with either labeled kallikrein or labeled esterase A2. The concomitant disappearance of free KBP-1 (TEBP-1) and the appearance of free TEBP-2 in the urine of old, hypertensive, proteinuric S rats suggests that: 1) most of the KBP-1 (TEBP-1) is bound to enzyme(s) in old rats; or 2) KBP-1 (TEBP-1) is largely converted to TEBP-2 in old rats; or 3) both are true and that binding of KBP-1 (TEBP-1) to enzymes is associated with the generation of TEBP-2.