Five experimentally naive female guinea pigs, all young adults, were observed during a succession of 11 experiments for visually evoked electrocortical responses (VERs). In each experiment, VERs were recorded within minutes after an animal had received a hyperthermalizing dose of microwave radiation in a 2450-MHz field. Latency of the N1 component of the VER, an inverse index of conduction velocity of primary visual fibers, was recorded via an electroencephalograph on a signal-averaging computer. Cortical and rectal temperatures were observed and recorded during measurement of VERs. The mean latency from onset of photic stimulation to the N1 peak diminished from 42.8 to 37.7 ms at respective cortical temperatures of 37.0 and 40.5°C, which represents a mean increase of conduction velocity of 3.4 percent per incremented Celsius degree. While cortical and rectal temperatures were highly correlated, the former were invariably lower by nearly 2°C, although they diverged slightly at higher elevations. Above a cortical temperature of 41.5°C, evidence of reversible and irreversible thermal toxicity was observed.