Renal blood flow is reduced in patients with chronic respiratory failure caused by chronic obstructive pulmonary disease (COPD), and changes in renal hemodynamics are likely to be important in the pathogenesis of the edematous state of cor pulmonale. We therefore examined the hypothesis that this renal vasoconstriction is reversible by comparing the effects of oxygen therapy and the renal vasodilator dopamine on renal hemodynamics in both hypoxemic patients with COPD and those who were also hypercapnic. We assessed renal hemodynamics noninvasively with color-flow Doppler ultrasound. In order to validate the technique we recorded renal hemodynamics in a group of healthy volunteers before and during a dopamine infusion, and in a subgroup we simultaneously measured effective renal plasma flow (ERPF) with para-aminohippurate clearance. In the healthy volunteers there was a 22.5% rise in time-adjusted mean arterial velocity (Tamx) measured by Doppler compared with a 22% rise in ERPF with dopamine. This rise was significant (p < 0.05). In hypoxemic, normocapnic subjects Tamx rose by 25% with oxygen (p < 0.005), 20% with dopamine (p < 0.005), and 24% with both therapies. There was no significant change in aortic flow whether the subjects received air, oxygen, or dopamine (p = 0.77). In the hypercapnic patients there was no significant change in aortic or renal velocities while receiving oxygen or dopamine (p = 0.85 and 0.86). We conclude that color-flow Doppler velocity measurements can accurately detect changes in renal blood flow. Oxygen and dopamine are equipotent renal vasodilators in hypoxic COPD, but the effect is not additive.(ABSTRACT TRUNCATED AT 250 WORDS)