Blood pressure, renal blood flow, glomerular filtration rate and urine composition and flow rate were measured, and renal vascular resistance was calculated, before and during infusion of prostacyclin (PGI2) i.v. or i.a. just proximal to the origins of 250 ng/min, PGI2 caused a significant but slight (13%) reduction in renal vascular resistance, but did not consistently increase renal blood flow. Given at the highest infusion rate which could be sustained without reducing blood pressure, PGI2 did not alter glomerular filtration rate or urine composition or flow rate. Intravenous PGI2 infusion caused reductions in systemic blood pressure and renal blood flow at a dose of 50 ng/min; infusion into the abdominal aorta produced equal reductions in pressure and flow at a dose of 500 ng/min. Although PGI2 is the only prostaglandin shown to reduce renal vascular resistance in the rat, it does not appear to be sufficiently specific for the renal vasculature or potent enough to justify suggesting a physiological role in regulation of renal function in the rat.