1. The influence of hypoventilation or hyperventilation on blood pressure and pulse rate responses to verapamil and nifedipine was studied in chloralose-anaesthetized rats. 2. Artificial ventilation with room air at a fixed volume of 10 ml kg-1 successfully induced combinations of hypoxaemia, hypercarbia and acidosis at a ventilator rate of 37 strokes min-1 and of hyperoxaemia, hypocarbia and alkalosis at 160 strokes min-1. 3. Hypoventilation caused significant decreases in both the blood pressure and pulse rate, whereas hyperventilation produced significant increases in these parameters. 4. In the controls, intravenous injections of graded doses of either verapamil or nifedipine caused dose-dependent decreases in mean blood pressure. The effects on pulse rate were not marked. 5. The hypotensive effects of verapamil were significantly more intense in hyperventilated rats, whereas those of nifedipine were significantly less pronounced in hypoventilated animals. The hypoventilated rats exhibited a significant dose-dependent decrease in pulse rate in response to verapamil administration. 6. It is concluded that cardiovascular responses to verapamil, nifedipine and probably other calcium antagonists are altered in the presence of blood gas abnormalities.