The effects of hypoxia (10% O2) on the thermosensitivities of preoptic neurons were studied in urethanized rats and compared to the effects of hypercapnia (10% CO2). This was examined by regression of neuronal activity on preoptic temperature. During hypoxia, the slope of the regression line increased significantly in 8 (23%) of 35 warm-sensitive neurons and decreased in eight other neurons (P less than 0.05). During hypercapnia, the slope of the regression line decreased significantly in 7 (30%) of the 23 warm-sensitive neurons (P less than 0.05). No neuron was found that significantly increased the slope of the regression line. The effects of hypoxia on thermosensitivities (i.e. the slope of the regression line) of PO neurons differed from those of hypercapnia in chi-square analysis (P less than 0.05). Responses of the cold-sensitive neurons to hypoxia or hypercapnia did not generally differ from those of the warm-sensitive neurons. During hypoxia and hypercapnia, arterial blood pressure, respiratory frequency, heart rate, and EEG were recorded to examine their relations to neuronal activity. The present results indicate that the thermosensitivities of preoptic neurons are modified by both hypoxia and hypercapnia, but that hypoxic differ from hypercapnic effects.