Abstract The effect of aortic chemoreceptor stimulation on renal circulation was studied in mature fetal lambs. A cesarean section was performed with the use of chloralose anesthesia, and the fetus was delivered with the umbilical circulation intact. Catheterization of a carotid artery provided a direct recording of fetal systemic arterial blood pressure and heart rate. Fetal condition was monitored by intermittent pH, Po 2 , and Pco 2 measurements. A 10 c.c. external delay loop incorporating a vertical tube, a cannulated electromagnetic flowmeter, and a pressure transducer was placed between the proximal end of an interrupted femoral artery and the distal end of an interrupted renal artery. Direct recordings of renal artery pressure and flow as well as renal arterial pressure-flow curves were thus obtained. The aortic chemoreceptors were stimulated by minimal stimulating doses of cyanide injected through a catheter placed in the aortic arch via a carotid artery. Cyanide stimulation usually caused a marked bradycardia and rise in systemic arterial blood pressure within 15 seconds. In the renal circulation, vasoconstriction occurred in most cases within the same interval, as evidenced by both direct recordings of pressure and flow and pressure-flow curves. These effects were abolished or greatly reduced by cervical section of the vagus nerves. It is concluded that control of the renal circulation is present and active in the mature lamb fetus.