Ventilatory responses to CO2 and hypoxia were measured in four normal volunteers breathing 30-50 per cent N2O with and without added inspiratory resistance. CO2 response was measured by a steady-state technique, hypoxic response by a non-steady-state progressive technique. Added inspiratory resistance depressed ventilatory responses to both CO2 and hypoxia. N2O had no effect on CO2 response either with or without resistance. N2O depressed the ventilatory response to hypoxia without added resistance and further depressed the response measured with added resistance. It is thought that this was probably the result of selective depression of peripheral chemoceptor function by N2O.