We studied the relative rates of release of active and inactive renin by the kidney in anesthetized pigs. Renin concentration was determined in arterial and renal venous plasma as follows: (1) before and after stimulation of renin release with isoproterenol or furosemide, (2) after suppression of renin release by extracellular fluid volume expansion, and (3) after administration of propranolol or indomethacin. Inactive renin was activated by dialysis of plasma at pH 3.3 for 24 hours. Renin concentration was estimated by radioimmunoassay determination of angiotensin I after a 3-hour incubation with excess homologous renin substrate. Following isoproterenol, the release of active renin increased from 8 +/- 4 (SEM) to 58 +/- 34 ng/min, and inactive renin increased from 53 +/- 33 to 321 +/- 136 ng/min. Similarly, furosemide stimulated the release of both active and inactive renin. Both forms of renin were suppressed by propranolol or indomethacin. Although changes in renin release following volume expansion were not statistically significant, the direction of change for both forms of renin was similar. Following logarithmic conversion of the rate of release, the plot of active vs. inactive renin formed a straight line. Values for active renin as a percentage of the total renin in simultaneously drawn arterial and renal venous plasma samples were not different. Thus, under the conditions of these experiments, release of active and inactive renin appears to be controlled by similar mechanisms. Both stimulation and suppression of renin release result in parallel changes in release of the two forms. Data on relative amounts of active renin in arterial and renal venous plasma suggest that there is no systemic conversion of the two forms.