The respiratory depressant effects of 1.0 and 1.5 mg/kg of tramadol were studied in 14 healthy young persons by determining the CO2 response (Read's method) and the mouth occlusion pressure (Whitelaw's method) before and 10, 30 and 60 min after administration of the drug. The decrease in minute volume VE (PACO2 = 55 mmHg), dependent on dose, ranged between 11,2 and 17,8%. The mean ventilatory response to CO2 decreased after administration of 1.0 and 1.5 mg/kg of tramadol by 15.4 and 25.4% respectively of the normal value. In the same patients, the mouth occlusion pressure responses decreased by 16.1 and 25.6% respectively. There were no significant changes in mean inspiratory flow rate (determined from tidal volume and averaged time of inspiration). These findings demonstrate that tramadol, in contrast to morphine, produces less depression of the ventilatory responses to CO2 in healthy subjects.