1. Prostaglandin E- and F-like material has been estimated in renal venous blood of the left kidney of anaesthetized rabbits following renal nerve section. Prostaglandins were estimated by bioassay following solvent extraction and column chromatography. 2. Electrical stimulation of the renal nerves of the left kidney to reduce renal blood flow by approximately 15% for 15 min resulted in a significant increase in the concentration of prostaglandin E-like material in the renal venous blood. The peak values were normally seen either in the last 5 min of the stimulation period or in the first 5 min after the end of the stimulation period. The concentration of prostaglandin F-like material was not significantly altered. 3. Similar reduction of renal blood flow of the left kidney by renal artery constriction also resulted in a significant increase in the concentration of prostaglandin E- but not F-like material in renal venous blood. The timing and magnitude of the response was comparable with that observed with renal nerve stimuation. 4. The effect of an angiotensin I converting enzyme inhibitor, SQ 20881, on the response to both renal nerve stimulation and renal artery constriction has been studied. The administration of the drug did not significantly reduce the release of prostaglandins from the denervated kidneys, however, the increase in prostaglandin E-like material, in response to both stimuli, was abolished. 5. The results suggest that the increase in prostaglandin E-like material released from the kidney in response to low frequency stimulation or to modest reductions in renal blood flow is dependent on the release of renin and that the effect is mediated by the formation of angiotensin II and not angiotensin I.