Ventilatory responses to CO2and hypoxia were measured in four normal volunteers breathing 30–50 per cent N2O with and without added inspiratory resistance. CO2response was measured by a steadystate technique, hypoxic response by a non-steadystate progressive technique. Added inspiratory resistance depressed ventilatory responses in both CO2and hypoxia. N2O had no effect on CO2response 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.