Oxygen toxicity of the central nervous system (CNS) can occur as convulsions and loss of consciousness, with no warning symptoms. A quantitative study of the effect of metabolic rate on sensitivity to oxygen toxicity was made in the rat. A group of 19 rats were exposed (126 exposures) to 12 combinations of four pressures (456, 507, 608 and 709 kPa) and three ambient temperatures (15, 23 and 29 degrees C) until the appearance of the first electrical discharge (FED) preceding clinical convulsions. Carbon dioxide production (VCO2) was also measured. A thermoneutral zone (mean VCO2 0.87 ml x g(-1) x h(-1)) existed between the temperatures of 24 and 29 degrees C; at temperatures lower than this, the metabolic rate increased by 1.2 to 4 times the resting level. Latency of FED decreased linearly with the increase in VCO2 at all four oxygen pressures. The slopes (absolute value) and intercepts decreased with the increase in oxygen pressure. This linear relationship made possible the derivation of an equation which described latency of the FED as a function of both oxygen pressure and metabolic rate. Various environmental and other physiological factors that have been said to influence sensitivity to CNS oxygen toxicity, enhancing the effect of the partial pressure of oxygen, can be explained by their effect on metabolic rate. It is suggested that in situations where there is a risk of oxygen toxicity of the CNS, that risk would be reduced by a lower metabolic rate.