The effects of fenoldopam, a selective dopamine-1 agonist, on regional blood flow and vascular resistance were examined in conscious unrestrained spontaneously hypertensive rats (SHR). Rats were instrumented chronically with pulsed Doppler flow probes to allow measurement of renal, mesenteric and hindquarters blood flow. Maximal changes in mean arterial pressure, heart rate and regional blood flow were recorded after i.v. administration of fenoldopam (1-1000 micrograms/kg). Fenoldopam produced a dose-dependent reduction in arterial pressure and increased heart rate in the conscious SHR. Significant increases in mesenteric (maximal = 69 +/- 10%) and renal (maximal = 42 +/- 4%) blood flows were observed at all doses of fenoldopam. In the hindquarters, vascular resistance was increased after low doses of fenoldopam (1-30 micrograms/kg), but decreased with higher doses (100-1000 micrograms/kg). After ganglionic blockade, hindquarter vasodilation was observed with fenoldopam at low (10 micrograms/kg) and high (500 micrograms/kg) doses. Pretreatment with metoclopramide (20 mg/kg) or SCH 23390 (30 micrograms/kg), a new selective dopamine-1 antagonist, significantly attenuated the vasodilator responses to fenoldopam in all three vascular beds. Pretreatment with propranolol failed to alter the vascular effects of fenoldopam, but reduced the tachycardia markedly. This study indicates that fenoldopam decreased regional vascular resistance in the renal, mesenteric and hindquarters vascular beds of the conscious SHR with the mesenteric vascular bed demonstrating the greatest reactivity. The vasodilation induced by fenoldopam in these vascular beds appeared to be due to stimulation of vascular dopamine-1 receptors.