The pulmonary vascular effects of dopamine and of dobutamine have been reported variably in the literature. We investigated the effects of dopamine and of dobutamine, at doses of 10 and 20 micrograms/kg/min, on the relationships of overall mean pulmonary arterial pressure (Ppa) to cardiac index (Cl) in 14 dogs ventilated alternatively in hyperoxic (FIO2, 0.4) and in hypoxic (FIO2, 0.1) conditions. Five-point Ppa/Cl plots were constructed by opening an arteriovenous femoral fistula or by stepwise inflations of a balloon in the inferior vena cava. These Ppa/Cl plots were rectilinear in all experimental conditions. Hypoxia was associated with an increase in Ppa over the entire range of Cl studied (2 to 5 L/min/m2). A deterioration in arterial oxygenation and an increase in O2 consumption constantly occurred after dopamine as well as after dobutamine administration. At 10 micrograms/kg/min (n = 6 dogs) neither drug affected Ppa over the entire range of Cl at both 0.4 and 0.1 FIO2. At 20 micrograms/kg/min (n = 8 dogs), dopamine and dobutamine increased Ppa at the lowest Cl (2 to 4 and 2 to 3 L/min/m2, respectively) at 0.4 FIO2, and attenuated hypoxia-induced increases in Ppa over the entire range of Cl. Two repetitions of alternated 0.4 and 0.1 FIO2 exposures had no effect on Ppa/Cl plots in 6 additional dogs given no drug. We concluded that at dosages as great as 20 micrograms/kg/min, as generally given in clinical practice, dopamine and dobutamine exerted similar effects upon the pulmonary circulation of intact dogs; either no change or an increase in hyperoxic pulmonary vascular tone and either no change or an attenuation of hypoxic pulmonary vasoconstriction.