Renal cortical slices from rat kidney were found to contain and secrete six forms of renin (EC 3.4.00.19). The multiple forms were demonstrated by an isoelectric focusing technique utilizing a stable, steady state pH gradient. The isoelectric points of the six secreted forms had the following pH values: 5.70, 5.55, 5.30, 5.15, 4.9, and 4.8, and represent, respectively, 1%, 62%, 17.5%, 8.5%, 8%, and 3% of the secreted renin. In the presence of isoproterenol (10-6 M), renin secretion from the renal cortical slices was increased significantly (87%) over control (P < 0.05). The increased renin secretion was accompanied by a significant change in the relative proportions of the six secreted forms of renin. These results indicate that multiple renin forms are stored and secreted by the rat kidney and that stimuli that increase renin secretion can preferentially increase the secretion of some of the multiple forms. It is hypothesized that isoproterenol stimulates the secretion of stored renin, which appears to be composed of a subset of the multiple renin forms.