The effects of ethanol on blood rheology were studied in twelve healthy male volunteers each serving as his own control. They drank 1.5 g (33 mmol) of ethanol per kg body weight in fruit juice over 2.5 h under controlled laboratory conditions. Blood and plasma viscosity, packed cell volume and erythrocyte flexibility were measured before and 3, 12 and 16 h after ethanol ingestion began, and again during a second session equal volumes of fruit juice were consumed. Packed cell volume varied similarly after ingestion of juice with or without ethanol. Blood viscosity remained almost unchanged after ethanol, while drinking of juice caused an initial decrease followed by a significant (P less than 0.05) increase from 4.04 (SEM 0.06) to 4.32 (SEM 0.18) mPa.s. The juice also caused more significant fluctuations in erythrocyte rigidity (whole blood) than ethanol, which had parallel but insignificant effects. We conclude that a blood ethanol concentration of 28 mmol l-1 failed to induce any marked changes in blood rheology.