The effects of hypoxia on ventilation and cerebral activity were studied in urethane-anaesthetized newborn guinea-pigs. Ventilation was measured by a pneumotachograph, and cerebral activity by a cerebral function monitor (CFM). All animals were subjected to either 9% O2 or 6% O2 in N2 for 10 minutes or until apnoea occurred. Hypoxia produced a biphasic response in ventilation, that is, an increase followed by a decrease. The initial increase was attributed to the elevation of the respiratory rate, whereas the tidal volume showed a pure decline. The respiratory rate reached its peak at 3 minutes of hypoxia (170 +/- 12% during 9% O2 and 169 +/- 12% during 6% O2). Cerebral activity during both 9 and 6% O2 breathing showed a small increase followed by a decrease. In the group subjected to 9% O2 the maximum CFM activity increased to 114 +/- 8% of the control level and the minimum activity increased to 113 +/- 7%, while in the group subjected to 6% O2 the maximum CFM activity increased to 104 +/- 5% and the minimum CFM activity to 101 +/- 3%. The depression of CFM activity was more pronounced with 6% O2 than with 9% O2. Regression analysis showed a linear correlation between ventilation and cerebral activity during both 9 and 6% O2 breathing. The results suggest that hypoxic ventilatory depression may be the consequence of cerebral depression produced by acute severe hypoxia.