Diaphragmatic function estimated by transdiaphragmatic pressure (Pdi) was studied in eight normal subjects during progressive partial paralysis with d-tubocurarine (dTc). Dynamic Pdi was measured during quiet tidal breathing, maximum deep inspiration, and 12-s maximum voluntary ventilation (MVV). Maximum static transdiaphragmatic pressure (Pdimax) was also measured during maximum static inspiratory efforts at four lung volumes. The maximum effect of dTc at a cumulative dose of 0.2 mg/kg abolished head-lift and handgrip ability. Pdimax at functional residual capacity was decreased to 42% of its control value indicating significant diaphragmatic weakness at this level of curarization. The weakness had no inpact on quiet breathing and a moderate effect on maximum inspiration. In either case Pdi represented an increasing fraction of Pdimax. MVV fell significantly before the Pdi during the maneuver decreased. This decreased MVV in curarized subjects reflects upper airway obstruction caused by pharyngeal muscle weakness and the diminished contribution of the other respiratory muscles that are important at high levels of ventilatory effort but more sensitive to effects of dTc.