Persistent vasospasm of the digits is difficult to attenuate or reverse. We studied the effect of intra-arterial nitroprusside on humoral and neurogenic digital vasoconstriction. Fingertip blood flow (FBF) was measured by venous occlusion plethysmography and capillary flow (CF) by the disappearance rate of a local injection of radioisotope. Small doses of nitroprusside (1 to 2 micrograms/min) increased FBF in fingers vasoconstricted by intra-arterial norepinephrine (4.7 +/- SE 1.7 to 38.4 +/- 26 ml min-1 100 ml-1 of tissue). Large doses of nitroprusside (up to 40 micrograms/min) did not increase FBF (11.8 +/- 6.0 ml to 6.6 +/- 3.9 ml) in fingers vasoconstricted by sympathetic nerve stimulation (body cooling). Forearm blood flow was measured in six subjects in the cool room and all showed an increase in flow with nitroprusside (6.7 +/- 1.1 ml to 26.0 +/- 6.0 ml), demonstrating that effective doses were used. Nitroglycerin (2.5 to 5 micrograms/min) significantly increased FBF from 5.1 +/- 1.9 ml to 14.8 +/- 6.9 ml as did phentolamine (50 to 100 micrograms/min) from 6.7 +/- 2.8 ml to 36.0 +/- 11.3 ml in sympathetically vasoconstricted subjects. Only phentolamine increased CF significantly (0.8 +/- 0.2 ml to 2.8 +/- 0.6 ml). Nitroprusside is an effective vasodilator for humorally induced vasoconstriction, but sympathetic digital vasoconstriction is resistant to nitroprusside and nitroglycerin is less effective than phentolamine. Phentolamine is the preferred agent because it increased FBF and CF.