A high prevalance of sleep disorders and sleep apnea syndrome in hemodialysis (HD) patients has been known for 10 years. Acetate, the buffer once most commonly used, favors intradialytic hypoxemia through hypoventilation and ventilation-perfusion changes. The aim of the present study was to assess the influence of buffer, acetate or bicarbonate, on sleep and ventilation during the night subsequent to an afternoon (2-7 p.m.) dialysis session. Ten patients, 8 males and 2 females, aged 35-71 years, dry weight 55-72 kg, on dialysis 15 h a week for 6-67 months, were randomly assigned first to acetate or bicarbonate, then to the other mode of treatment. After a series of six sessions using the same buffer, polysomnographic recordings from 9.00 p.m. to 6.00 a.m. were obtained. Sex, age, weight, data of first dialysis, blood pressure and sleep disorder-related symptoms were not correlated with the sleep apnea syndrome. Prolonged or important oxygen desaturations were never observed. Central apnea occurred more frequently during the night following acetate dialysis: x = 33 (0-180) versus 3 (0-15), p < 0.05. Obstructive apneas were not different. A defective modulation of ventilatory control after acetate HD might be held responsible for central apnea, which would constitute one more case for a widespread use of bicarbonate HD.