The effects of progressive hypoxia, obtained by decreasing FIO2 from 0.21 to 0.12, on arterial blood gases and acid-base balance were studied in 13 awake rats and 13 rats anaesthetized with halothane (inspired concentration 1.1%). The effects on ventilation of the decrease in FIO2 from 0.21 to 0.12 were studied in eight rats, awake and then anaesthetized. Halothane causes a decrease in PaO2 and an increase in PaCO2; it abolishes the ventilatory response to hypoxia. The effects of hypoxia on PaCO2 were identical in awake and in anaesthetized rats. In the awake rats, PaO2 decreased from 90.3 +/- 5.9 mmHg to 42.3 +/- 3.6 mmHg, and PaCO2 decreased from 36.7 +/- 3.3 mmHg to 28.1 +/- 1.8 mmHg. In the anaesthetized rats, PaO2 decreased from 78.8 +/- 6.2 mmHg to 34.8 +/- 4.2 mmHg, and PaCO2 decreased from 40.7 +/- 2.8 mmHg to 31.9 +/- 3.7 mmHg. The decrease in PaCO2 during acute hypoxia in the anaesthetized rat could be explained by a decrease in CO2 production, secondary to a decrease in oxygen consumption due to the metabolic and circulatory effects of halothane and hypoxia.