To determine the effects of endotracheal intubation on airway mechanics in healthy individuals, measurements of pulmonary function were made while semirecumbent subjects performed dynamic respiratory maneuvers. Eight healthy male volunteers were studied under three test conditions: 1) breathing through a 25-mm ID mouthpiece (control); 2) with an 8.0-mm ID endotracheal tube adaptor in the mouthpiece (external resistance); 3) with an 8.0-mm ID endotracheal tube 25 cm long in place. Decreases in peak inspiratory flow and peak expiratory flow during flow–volume loops were the only significant changes seen with the external resistance. Peak flows were also decreased by intubation, but to a significantly greater extent (P< 0.01). Forced vital capacity (FVC) decreased significantly with tracheal intubation to 89 ± SEM 2 per cent of control. Forced expiratory volume in 1 sec (FEV1) was also decreased significantly, whether expressed as actual volume or as FEV1/FVC. Compliance of the lung was unchanged after intubation, but flows below mid-vital capacity during maximal expiratory flow-volume (MEFV) curves were decreased significantly from control. Responses to breathing helium were abnormal with intubation in many subjects, although mean responses did not differ significantly from control. Mean values for closing volume and slope of Phase III (ΔN2per cent/1) were likewise not significantly different from control, but in at least three subjects were indicative of peripheral airway obstruction. The decreased peak flows and FEV1with intubation reflect the tube's behavior as a significant fixed upper-airway resistance. The decreased FVC and expiratory flows below mid-vital capacity indicate that with intubation many subjects showed diffuse airway constriction superimposed on the fixed resistance of the tube.