The effects of neuropeptide Y (NPY, 10 micrograms/kg bolus iv) on cardiac output, renal blood flow and myocardial contractility were determined in intact renal hypertensive and normotensive rabbits instrumented with ultrasonic flow transducers or left ventricular catheters. The basal plasma concentration of NPY-like immunoreactivity in arterial blood was greater in the hypertensive rabbits (4.2 +/- 0.7 micrograms/l) than in normotensive animals (2.2 +/- 0.4 micrograms/l, p less than 0.05). There were similar moderate increases in arterial blood pressure and total peripheral resistance following NPY, but a small NPY-induced reduction in cardiac output in normotensive rabbits was not seen in hypertensive animals. Resting peak left ventricular dP/dt (an index of myocardial contractility) was higher in hypertensive rabbits (7397 +/- 619 vs 5551 +/- 342 mmHg/sec, p less than 0.05), but there was no significant difference between the maximum NPY-induced falls in peak dP/dt. NPY produced significant peak reductions in renal blood flow in both hypertensive (from 2.5 +/- 0.2 to 1.2 +/- 0.2 kHz, p less than 0.05) and in normotensive rabbit groups (from 2.2 +/- 0.1 to 0.3 +/- 0.1 kHz, p less than 0.05), but the fall in renal blood flow and the corresponding rise in renovascular resistance were smaller in the hypertensive animals (p less than 0.05). The cause of this apparent decrease in renovascular reactivity in the renal hypertensive model was not determined.