We measured nocturnal plasma erythropoietin concentration (EPO) in eight subjects with obstructive sleep apnea syndrome (OSAS) and eight healthy overweight subjects while they were undergoing nocturnal polysomnography. We also measured EPO (radioimmunoassay) after 120 min of exposure to 10.5% O2. The subjects with OSAS had a respiratory disturbance index (RDI) of 50.8 +/- 41.9 and a maximal O2 desaturation of 65 +/- 13.3%, and they spent 104.5 +/- 89.3 min out of a total sleep time of 356 +/- 54 min below 90% oxygen saturation (T90). Nocturnal EPO concentrations were normal and did not differ between the two groups. We found no correlation between the T90, T80, or RDI and either the mean or maximal EPO concentrations. After the exposure to 10.5% oxygen, during which oxygen saturations were between 80 and 85%, the healthy subjects and those with OSAS developed increases in EPO of 34 and 49%, respectively, 240 min after the initiation of exposure (p < 0.05). We conclude that the nocturnal hypoxemia occurring in these subjects with OSAS may not have been sufficiently severe to stimulate an increased EPO production. The lack of EPO response was not due to down-regulation of EPO production.