The effects of diazepam (0.14 mg/kg) on steady-state gas exchange were studied in 13 patients, and the responses to a carbon dioxide stimulus using a recently introduced rebreathing technique are described. There were significant increases in VD/VTand Paco2, and a decrease in VT. &OV0312;Eand &OV0312;o2- were unchanged. In more than half of the patients there was a depression of the slope of the ventilatory response to CO2, or a depression of the &OV0312;E57, or both.The evidence indicates that diazepam depresses the ventilatory response to CO2, and also modifies the characteristics of gas exchange within the lung, producing a rise in VD/VT, and often associated with a rise in Pactco2.The variation in incidence and severity of changes in respiratory function among individuals is evidenced in this study of the effects of diazepam.