1. The effects of different energy substrates, of low temperature, of urea, and of ouabain and ethacrynic acid were studied on the rate of renin release from viable juxtaglomerular cells during superfusion of isolated rat glomeruli. 2. Neither lactate nor glutamate altered renin release rate from that observed using glucose as the sole energy substrate. Succinate 10 mM elevated release transiently but did not influence the release caused by reductions in osmolality through lowering sucrose concentration. 3. Peak renin release was more prolonged and returned more slowly to control following reductions in osmolality in phosphate‐Ringer than in bicarbonate‐Ringer. 4. At 37 degrees C, the peak of renin released induced by hypo‐osmolality was smaller and delayed, and returned earlier to control than at 30 degrees C. Reduction in temperature from 30 to 4 degrees C resulted in a 32‐fold increase in basal release rate. At 4 degrees C a 20 m‐osmole/kg reduction in tonicity caused an additional 2‐5‐fold increase in release rate. 6. Increasing superfusate osmolality with urea did not affect basal renin release but 100 mM urea suppressed the releasing effect of a 15 mM reduction in NaCl concentration. 7. Ouabain (10(‐4) M) caused a small (33 +/‐ 9%, P less than 0‐025) transient increase in renin release. Ethacrynic acid (10(‐3) M) provoked a progressive increase in release reaching 100 +/‐ 15% above control within 50 min. In the presence of both inhibitors the release provoked by hyposmolality was prolonged. 8. It is concluded that renin release in vitro is a function of actively regulated cell volume and it is proposed that a similar mechanism could underline both barorecptor and macula densa controls of renin secretion in vivo.