The effects of alcohol intake on haemodynamics and heart rate variability were investigated with relation to genotypes of aldehyde dehydrogenase 2 (ALDH2), which were determined in 33 male Japanese volunteers (mean +/- s.e., 35.7 +/- 1.4 years) using the PCR-RFLP method. On the alcohol intake day, they consumed 660 ml of beer containing 33 ml of ethanol (0.3-0.5 g/kg of body weight) from 18.00 to 18.30. On the control day, they ingested the same amount of non-alcoholic beer. Ambulatory blood pressure, heart rate, and ECG R-R intervals were measured during a 24-h period with a portable recorder. A power spectral analysis of R-R intervals was performed to obtain the low-frequency (LF) and high-frequency (HF) components. Sixteen subjects were homozygotes for the normal ALDH gene (active ALDH2), only one was a homozygote for the mutant ALDH2 gene (inactive ALDH2), and the remaining 16 were heterozygotes (inactive ALDH2). Alcohol intake did not change 24-h average blood pressure (BP) either in the active ALDH2 group or in the inactive ALDH2 group. However, during the time interval from 18.30 to 0.00, alcohol intake significantly decreased diastolic BP in the active ALDH2 group and both systolic and diastolic BPs in the inactive ALDH2 group. In the active ALDH2 group, alcohol intake did not change heart rate, while in the inactive ALDH2 group, alcohol intake significantly increased 24-h average heart rate by 5.3 +/- 1.6 beats per minute (P < 0.01). In the active ALDH2 group, neither the LF nor the HF component was changed by alcohol intake, while in the inactive ALDH2 group, both the LF and the HF components were significantly decreased during the time interval from 18.30 to 0.00. These results demonstrate for the first time that ALDH2 genotypes modify the effects of intake of a small amount of alcohol on haemodynamics and heart rate variability in Japanese men.