In order to measure the hypercapnic ventilatory response (HCVR) in experimental animals, it is desirable that the animals be unanesthetized and unrestrained. In the present study, we used a barometric chamber to measure steady state HCVR of hamsters with chronic cannulation of their aortas. In 16 hamsters, the mean (+/- SD) of HCVR, defined as changes in ventilation divided by changes in Paco2 (range 53-73 mmHg), was 10.1 +/- 4.9 ml X min-1 X mmHg-1. The rise in ventilation during CO2 inhalation at low concentrations was due to a rise in mean inspiratory flow and tidal volume. As inhaled Pco2 increased, frequency of breathing increased because expiratory time fell progressively. Inspiratory time rose slightly with small increases in inhaled CO2, but returned to baseline values as the concentration of inhaled Pco2 increased. These changes in ventilation and its components appear to resemble those found in the unanesthetized cat and in man, but are somewhat different from those found in the rat.