The flash evoked potential (FEP) of rats has a large negative peak (N(sub 160)) approximately 160 msec following stimulation. The peak has been reported to be modulated by the subject's state of behavioral arousal and influenced by several test parameters. These experiments examine the influences of repeated testing, the number of stimuli/session, interaction of ambient illumination and flash intensity, and the effect of pupillary dilation on the development and amplitude of peak N(sub 160). The amplitude of peak N(sub 160) increased with daily testing, and reached an asymptotic amplitude by about day 10. This amplitude was affected by the intensity of the flash stimulus relative to the ambient illumination (RFI), and appeared to reach a ceiling amplitude at greater than 50 dB RFI. The number of stimuli/session and dilation of the subject's pupils did not have a large influence on the growth or asymptotic level of peak N(sub 160) amplitude. The data are consistent with the hypothesis that the growth of peak N(sub 160) may represent a sensitization-like phenomenon.