The release of newly synthesized renin by dog renal cortical slices was investigated. A radioactive protein believed to be renin was secreted from cortical slices into an incubation medium containing radioactive leucine. The nature of the radioactive protein was established by dialyzing the incubation medium after the cortical slices were removed, and subsequently applying the dialysate to 15% discontinuous polyacrylamide gels. Renin activity in the gels comigrated with the labeled protein. The labeled protein, as evidenced by a peak in 3H or 14C disintegrations per min, also comigrated with renin activity in isoelectric focusing gels. The molecular weight of the labeled protein was found to be the same as renin, approximately 40, 000. Cortical or medullary slices possessing little or no renin activity did not secrete the labeled protein into the incubation medium. Isoproterenol significantly increased total renin secretion from the slices, but not the secretion of newly synthesized renin. The data support the hypothesis that two or more pools of renin are available for release. One pool releases newly synthesized renin, while another pool releases older, possibly stored renin, dependent in part on the stimuli available for renin release.