The effects of different protein regimens on pulmonary gas exchange and ventilatory drive were examined in eight postoperative patients receiving inspiratory pressure support ventilation. They were studied during 60 consecutive hours, which included two 12-h periods of high protein intake (33%) of total caloric intake provided as protein), each of them being preceded and followed by a 12-h period of standard protein intake (14% of total caloric intake provided as protein). Throughout the study, total caloric intake was 1.5 times the predicted resting energy expenditure. Nitrogen was provided as a 24% branched chain amino acid (BCAA) solution during the period of standard protein intake. During the periods of high protein intake, it was provided as a 24% and a 41% BCAA solution. Pulmonary gas exchange was continuously measured during the second half of each period, with the use of a mass spectrometer system. Measurements of the ventilatory response to CO2 (FICO2 0, 1.5, and 3%) were achieved at the end of each dietary regimen. O2 consumption, CO2 production, respiratory quotient, minute ventilation, and PaCO2 were the same for the three protein regimens. Changing protein intake failed to affect the ventilatory response to CO2. The authors conclude that, in postoperative patients having inspiratory pressure support ventilation, the administration of a high protein intake does not affect the ventilatory drive and the pulmonary gas exchange.