Immediate circulatory reactions to acute intragastric ethanol administration were studied by a catheterization technique in spontaneously breathing dogs. Diluted ethanol was given in a dosage of 1 g/kg in test group I (n = 11), and 2/kg in group II (n = 10). The control group (m = 14) received only water. The highest blood ethanol concentration was 0.90 +/- 0.07 mg/ml (mean +/- SE) in group I, and 1.97 +/- 0.10 mg/ml in group II. Heart rate and cardiac output increased (p less than 0.,001), but stroke volume, mean aortic blood pressure and right atrial blood pressure remained practically unchanged. Systemic vascular resistance decreased. Mean pulmonary arterial blood pressure increased markedly in both test groups (p less than 0.001) while pulmonary arterial wedge pressure did not change. The pulmonary arterial resistance increased (p less than 0.01). Changes in respiratory rate or volume and arterial pO2 were negligible in group I, but respiratory minute volume decreased in group II. In conclusion, ethanol in concentrations 0.5 to 2.0 mg/ml increased resistance in the pulmonary arterial tree, indicating pulmonary arterial vasoconstriction, but reduced systemic vascular resistance, thus putting a concept of peripheral vasodilation in favour.