1. The coincident release of renin and lactic dehydrogenase (LDH) from rat renal cortical tissue slices was studied during calcium depletion, metabolic inhibition, and with the addition of ouabain (1 mM) to the incubation medium.2. The results indicate that although LDH accumulated in the medium during incubation, the pattern was dissimilar to that of renin. Ouabain significantly inhibited renin release in calcium containing medium, but had no effect on LDH release. Renin release was potentiated in calcium ;free' media, while calcium depletion reduced the release of LDH.3. The addition of potassium cyanide (2 mM) significantly inhibited the release of renin from these tissue slices. Cyanide was ineffective, however, when administered in calcium ;free' medium.4. At reduced incubation temperatures (5 degrees C) the release of both renin and LDH were significantly reduced.5. Medium sodium depletion caused a significant inhibition of renin release. The simultaneous removal of calcium from the medium did not restore renin release to control levels.6. These results are not consistent with the hypothesis that spontaneous renin release during calcium depletion and metabolic inhibition is a result of cell enlargement and increased membrane permeability. On the other hand, the in vitro release of renin during these experiments appeared to be inversely related to the intracellular concentration of calcium.