d-Tubocurarinc (dTe) was administered intravenously to six healthy unanesthetized volunteers to assess the effects of partial paralysis on ventilatory response to CO2. Each subject received during a −40-minute period 0.2 mg/kg. consisting of five incremental doses at intervals 10 minutes apart. Isohypereapnia with PETCO26–7 torr above each subject's resting level was maintained throughout>dTc administration. Ventilation at this level of stimulus was 23.8 ± 1.1 I/min (mean ± SE) before administration of>dTc. about three times resting levels. Steady-state minute ventilation measured during the period 4–6 minutes after each dose of>dTc failed to decrease significantly; the levels of ventilation were maintained principally by increased respiratory frequency, since tidal volumes declined significantly from an average of 1,550 ml to 1,050 ml (P> 0.025). Changes in the slope of the CO2-response curve varied widely among subjects. Although the control slope of 2.65± 0.76 1/min/torr (mean ± SE) was reduced to 1.50 ± 0.36 1/min/torr after partial curarization. the change was not significant (P< 0.10). Ventilation was maintained at a time when grip strength was 6 per cent of control, vital capacity was 52 per cent of control, and maximum static respiratory pressures were 35–40 per cent of control. Nevertheless, the results suggest significant impairment of vital respiratory functions such as coughing, deep breathing, and the ability to maintain a patent airway in the absence of endotracheal intubation.