20 isolated dog kidneys were perfused in vitro at 37 degrees C during 6h in a closed circuit system with a Roller pump maintaining a pulsatile flow with a constant output. The perfusate electrolyte concentrations were kept constant. In 12 experiments where a hemodiluted standardized perfusion fluid was used, a significant (0.01 greater than p greater than 0.001) correlation (r = 0.798) was found in the beginning of the experiments between the mean arterial pressure expressed in mm Hg (y) and the renin level, expressed in log U/1 (x): y = 57.0+76.8x. In 4 other experiments, phenoxybenzamine added to the perfusion fluid decreased the arterial pressure but did not prevent the rise of the renin level. In 4 other experiments where the perfusion fluid did not contain blood, renin level and arterial pressure increased, whereas in the experiments using a circuit without a kidney, the renin level did not rise. The possible role of the renin-angiotensin system in causing increased renal vascular resistance during in vitro kidney perfusions is discussed.