The hemodynamic effects of 7 min i.v. sodium nitroprusside (NP) were studied in conscious dogs previously instrumented for measurement of arterial pressure, cardiac output, regional blood flow distribution, left ventricular (LV) pressure, and internal dimensions. Nitroprusside, 25 microgram/kg/min, reduced mean arterial pressure by 23 +/- 3%. Cardiac output increased initially by 39 +/- 7% and returned toward control by the end of the infusion. Regional blood flows increased initially; the relative rise was greatest in the coronary (+ 225 +/- 39%), intermediate in the mesenteric (+ 98 +/- 23%) and iliac (+ 38 +/- 6%), and least in the renal (+ 10 +/- 3%) bed. By the end of the infusion period the vasodilation was unchanged in the iliac bed, less intense in the coronary and mesenteric, while in the iliac bed, blood flow was reduced and resistance was actually increased by 33 +/- 11% above control. A generalized vasonconstriction ensued after cessation of infusion. In contrast, when the drug was administered intra-arterially to the iliac bed, arterial pressure did not fall and only iliac vasodilation was observed. Peak cardiac effects were characterized by increases in heart rate and LV dP/dt, along with marked reduction in LV end-systolic diameter (- 13 +/- 2%), and in end-diastolic diameter (-17 +/- 2%) and pressure. LV end-diastolic diameter fell even heart rate was maintained at a constant rate by pacing. Thus, in the conscious dog, NP reduced LV dimensions substantially, while inducing changes in peripheral beds. The differences in these effects depend on interactions between the direct effects of NP and the opposing effects of reflex adjustments which appear sufficiently powerful to result in net constriction of the iliac bed late during the infusion.