Acetylcholine's effect on the distribution of vascular resistance and compliance in the canine pulmonary circulation was determined under control and elevated vascular tone by the arterial, venous, and double occlusion techniques in isolated blood-perfused dog lungs at both constant flow and constant pressure. Large and small blood vessel resistances and compliances were studied in lungs given concentrations of acetylcholine ranging from 2.0 ng/ml to 200 micrograms/ml. The results of this study indicate that acetylcholine dilates large arteries at low concentrations (less than or equal to 20 ng/ml) and constricts small and large veins at concentrations of at least 2 micrograms/ml. Characterization of acetylcholine's effects at constant pulmonary blood flow indicates that 1) large artery vasodilation may be endothelial-derived relaxing factor-mediated because the dilation is blocked with methylene blue; 2) a vasodilator of the arachidonic acid cascade (blocked by ibuprofen), probably prostacyclin, lessens acetylcholine's pressor effects; 3) when vascular tone was increased, acetylcholine's hemodynamic effects were attenuated; and 4) acetylcholine decreased middle compartment and large vessle compliance under control but not elevated vascular tone. Under constant pressure at control vascular tone acetylcholine increases resistance in all segments except the large artery, and at elevated vascular tone the pressor effects were enhanced, and large artery resistance was increased.