1. The effects of endothelin infusion on renal vascular resistance (RVR), glomerular filtration rate (GFR) and the interaction with locally generated endothelium-derived relaxant factor (EDRF) were studied in the rabbit isolated perfused kidney (IPK). For comparison the effects of infusions of angiotensin II (AII) and noradrenaline (NA) were also assessed. 2. Each kidney was perfused at a constant rate of 10 ml min-1 and alterations in RVR determined by measuring changes in perfusion pressure. GFR was determined by the clearance of [51Cr]-EDTA, using timed urine collections. 3. Endothelin (10(-11)-10(-9) M) produced a dose-related increase in RVR. Endothelin was approximately 30 times more potent in molar terms than AII and 500 times more than NA at inducing a 50 mmHg increase in perfusion pressure. 4. Endothelin appeared to be a weak inducer of EDRF release in the IPK as EDRF inhibitors methylene blue (10 microM) or haemoglobin (10 microM) only slightly augmented the increase in RVR at a given concentration of endothelin. In contrast the effect of NA on RVR was significantly increased by methylene blue (10 microM) whereas that induced by AII was not affected. 5. Endothelin infusion produced a significant, dose-dependent decrease in GFR of the IPK, contrasting with an increase in GFR during AII infusion and a minimal effect of NA on GFR. This supports evidence that AII is predominantly a constrictor of effernt glomerular arterioles and that NA constricts both afferent and efferent glomerular vessels. We suggest that the vasoconstrictive effect of endothelin in the kidney is predominantly preglomerular, which explains its effect on GFR.