The electroretinogram (ERG) was measured in lightly anaesthetised rats exposed to 4-7 atm O2 until death. An initial increase and later decrease in amplitude with increase in latency was seen which, although not consistently related to the appearance of seizures characteristically seen above 4.5 atm 02, appeared closely linked to systemic factors. A fall in respiration rate always preceded the fall of the ERG, and apnoea preceded ERG extinction. When blood pressure and ERG were measured in two comparable groups of rats exposed to 5.76 atm O2, both records showed similar time courses. The cerebral visual evoked response (VER) was measured at 5.76 atm O2 simultaneously with the ERG. The later VER waves showed a degree of enhancement at the time of seizures, but otherwise amplitude and waveform were maintained until late in toxicity. EEG spiking gave the earliest indication of toxicity, and it is concluded that the primary projection to the rat visual cortex is comparatively resistant to high-pressure oxygen.