The microvascular effects of sodium nitroprusside (SNP) were studied in rat striated muscle to determine the peripheral vascular sites of action of the drug. Eleven male rats were anesthetized with pentobarbital and the cremaster muscle was prepared for television microscopy. The internal diameters of three arterioles and two venules of each animal were measured before) during and after SNP infusion (40 μg/kg/min). First-order arterioles (123 ± 7 μm) showed no significant response to SNP, and their internal diameters correlated poorly (r = −0.31) with decreases in mean arterial pressure (MAP). The internal diameters of third-order arterioles (42 ± 2 μm) correlated well with MAP (r = 0.85), but these arterioles did not dilate significantly during SNP infusion. The internal diameters of fourth-order arterioles (15 ± 1 μm) correlated best with MAP (r = 0.91), and significant (P < 0.05) dilatation occurred promptly during SNP infusion. Thereafter, the responses of fourth-order arterioles could be divided into two subgroups. One subgroup dilated initially but promptly returned to control diameters, while the other subgroup remained dilated (P < 0.05) throughout exposure to SNP. Both third- and fourth-order arterioles constricted significantly when SNP infusion was discontinued. Venular diameters were not affected by SNP. In five additional rats, topical application of SNP (10-4 M) to the cremasteric microvasculature resulted in dilatation of fourth-order arterioles, which persisted throughout exposure to SNP, but reverted to control diameters when the drug was removed. The transient dilatation of some fourth-order arterioles, and the arteriolar constriction observed when intravenous infusion of SNP was terminated, appear to represent compensatory responses to hypotension, since these effects were not seen with topical application of SNP. The results indicate that the primary peripheral vascular site of action of SNP is in fourth-order arterioles.