The contribution of inert gases to the risk of central nervous system (CNS) oxygen toxicity is a matter of controversy. Therefore diving regulations apply strict rules regarding permissible oxygen pressures (PO₂). We studied the effects of nitrogen and helium (0, 15, 25, 40, 50 and 60 %) and different levels of PO₂ (507, 557, 608 and 658 kPa) on the latency to the first electrical discharge (FED) in the EEG in rats, with repeated measurements in each animal. Latency as a function of the nitrogen pressure was not homogeneous for each rat. The prolongation of latency observed in some rats at certain nitrogen pressures, mostly in the range 100 to 500 kPa, was superimposed on the general trend for a reduction in latency as nitrogen pressure increased. This pattern was an individual trait. In contrast with nitrogen, no prolongation of latency to CNS oxygen toxicity was observed with helium, where an increase in PHe caused a reduction in latency. This bi-modal response and the variation in the response between rats, together with a possible effect of ambient temperature on metabolic rate, may explain the conflicting findings reported in the literature. The difference between the two inert gases may be related to the difference in the narcotic effect of nitrogen. Proof of a correlation between individual sensitivity to nitrogen narcosis and protection by N₂ against CNS oxygen toxicity may lead to a personal O₂ limit in mixed-gas diving.