To investigate the enzyme involved in the activation of plasma inactive renin in vivo, we measured the changes in plasma active renin, inactive renin and prekallikrein, and the levels of urinary kallikrein excretion in 10 primary glomerulonephritic patients before and after a low sodium (Na; 17 mEq/day) constant potassium (K; 40 mEq/day) diet for 5 days. Plasma inactive renin was activated by trypsin. Active renin was measured by the amount of angiotensin I generated when sheep substrate was added to the plasma. Plasma prekallikrein was measured by its activity on substrate S-2302 after activation. Urinary kallikrein was measured by its activity on substrate S-2266. The results showed that changes in plasma active renin (7.7 +/- 2.9 to 23.8 +/- 9.9 ng/ml/h), and inactive renin (61.5 +/- 10.2 to 145.7 +/- 53.9 ng/ml/h) and urinary kallikrein excretion (6.7 +/- 1.1 to 10.8 +/- 2.4 nkat) were significant. No significant change in plasma prekallikrein was observed. The correlation between plasma active renin and inactive renin was significant both before and after the low salt diet. The correlation between the ratio of active to total renin and urinary kallikrein was significant before the low salt diet. These results are compatible with the postulate that plasma inactive renin may be a renin precursor, but they do not support the theory that either plasma kallikrein or renal kallikrein is related to activation of inactive renin in vivo.