Renin release was studied during the infusion of catecholamines (.7–5.6 µg/min) and angiotensin II (.25–1.05 µg/min) into the renal artery, and also during and following iv infusion of 16 µg/min noradrenaline. Filtration rate (Ccr), RPF, and RBF were estimated from the renal clearance of creatinine and the extraction ratio. Marked increases in renal vein renin titer and V-A renin difference occurred within 1 min of the onset of infusion of catecholamine into the renal artery and continued for 15 min beyond the infusion period. This infusion was always accompanied by diminished renal function. During iv infusion of noradrenaline, renin release did not usually occur, but on stopping the iv infusion, the ensuing fall in blood pressure was coincident with a marked release of renin apparently unaccompanied by decreased renal function. Infusion of angiotensin into the renal artery, in spite of causing renal vasoconstriction and marked decrease in urine volume, was not accompanied by renin release. The difficulties of measuring renin release under these conditions are discussed.