The renal vasoconstrictor responses to graded frequencies of direct electrical renal nerve stimulation and renal arterial norepinephrine injections were evaluated before and during renal alpha-1 adrenoceptor blockade in anesthetized dogs. Renal alpha-1 adrenoceptor blockade was achieved by infusing increasing doses (1, 10, 20, 40 micrograms/kg/min) of prazosin, a selective postsynaptic alpha-1 adrenoceptor antagonist, into the renal artery. The renal vasoconstrictor response to norepinephrine (1.5 micrograms) was completely abolished by 10 micrograms/kg/min prazosin. However, the renal vasoconstrictor response to a frequency of renal nerve stimulation which produced less renal vasoconstriction than norepinephrine (1.5 micrograms) in the absence of prazosin was not abolished by either 10, 20, or 40 micrograms/kg/min prazosin. During prazosin administration, neither renal nerve stimulation nor norepinephrine produced renal vasodilatation. Therefore, at prazosin doses two- and fourfold greater than that demonstrated to produce complete renal alpha-1 adrenoceptor blockade, no renal vasodilator responses to graded renal nerve stimulation were observed. These studies provide no evidence in support of a neural renal vasodilator mechanism in the dog.