Flash visual evoked potential (VEP) and electroencephalogram (EEG) changes during the menstrual cycle were studied using healthy females having regular menstruation, with 21 at the follicular phase (FP) and 23 at the luteal phase (LP). The following results were obtained. (1) The waveforms of Group Mean VEPs of both groups had approximately similar triphasic contours, consisting of 16 components of P 1-N 8 up to 500 msec of latency. (2) Latencies tended to be longer in LP. (3) Interpeak amplitudes tended to be larger in LP, and one VEP interpeak amplitude (P 5-N 7) of long latency component was significantly larger at LP after eliminating the effect of body height by ANCOVA for 2 CH. (4) Quantitative analysis of EEGs between FP and LP resulted in a tendency for increased alpha, and decreased beta power % at LP. Since estrogen increases the VEP amplitude, and decreases the VEP latency and the alpha activity of EEGs, the large VEP amplitude, the tendency for prolonged VEP latency, and the tendency for increased alpha power % at LP observed in this study indicate that the VEP amplitude at LP reflects the effect of estrogen, and that the VEP latency and EEGs at LP reflect the effect of progesterone.