1. The acute effects of a moderate dose of ethanol (1g/kg body weight) on heart rate and blood pressure variability and baroreflex sensitivity were studied in 12 healthy male subjects in a juice-controlled experiment. Electrocardiographic and finger blood pressure data were recorded and stored in a minicomputer during 5 min of controlled breathing (15 cycles/min) and during deep breathing (5 s inspiration, 5 s expiration, four cycles) before drinking and hourly thereafter for 3 h. 2. Mean breath alcohol concentration rose to 18.9 mg/100 ml. In the time domain analysis, the root mean square difference of successive R-R interval decreased significantly with ethanol as compared with the juice experiment. The difference remained statistically significant even after adjustment for the shorter R-R interval after alcohol. In the frequency domain analysis the high-frequency (0.15-0.5 Hz) spectral power showed a significant decrease after alcohol intake. Also, the index of sensitivity of the baroreceptor reflex (square root of R-R interval power/systolic blood pressure power) decreased significantly in the high-frequency component. Ethanol did not change finger systolic blood pressure, and power spectral analysis did not show significant variability in blood pressure. 3. These data indicate that acute intake of moderate amounts of alcohol causes a significant decrease in heart rate variability owing to diminished vagal modulation of the heart rate.