A second-gas effect of nitrous oxide upon alveolar carbon dioxide tension (PAco2) has been demonstrated in decerebrated cats ventilated with a volume-limited respirator. The Increase in alveolar tension of carbon dioxide (the second gas) which is associated with the start of administration of nitrous oxide (the first gas) is the result of a concentrating effect produced by rapid absorption of nitrous oxide. At an equilibrated PAco2of 40 torr, administration of 80 per cent nitrous oxide resulted in an increase of PAco2of 2.63 ± 0.29 torr (&OV0335; ± SE). The higher the concentration of nitrous oxide and the lower the level of ventilation (the higher the equilibrated PAco2), the greater the increase in PAco2.