Various behavioral and neurological phenomena are reported to occur in relation to the phase of the menstrual cycle in women, including mood changes, suicide attempts and seizures. It is suggested that fluctuations in female sex hormones during the menstrual cycle might be related to these findings, and that the general state of arousal of the nervous system might vary with the menstrual cycle. In this study, eight women were studied on approximately the 3rd, 14th, 24th, 26th, and 28th days of two successive menstrual cycles. Four tests, which have been reported to reflect state of arousal, were performed at each session. These included galvanic skin potential, reaction time, two-flash threshold and time estimation. In addition, blood was drawn for plasma cortisol levels and a Nowlis Mood Ad-jeetive Check List was completed at each session. The data were analyzed in several ways. Analysis of variance for each of the four tests separately showed that reaction time and skin potential did not vary significantly over the menstrual cycle. Time estimation showed a significant variation over the menstrual cycle. Two-flash threshold showed a trend but was not statistically significant. Both time estimation and two-flash threshold showed a tendency toward higher values in the premenstrual period. The results for each of the tests were correlated with each other. Neither reaction time nor skin potential correlated with any of the other tests. However, there was a significant correlation between time estimation and two-flash threshold. Correlations between cortisol levels and two-flash thresholds, as well as correlations between cortisol levels and time estimation, were inconclusive. Self-ratings of concentration and social affection on the Mood Adjective Check List correlated negatively with time estimation. The behavioral fluctuations during the menstrual cycle may be related to a distortion of time sense, or to changes in the state of arousal, which may be secondary to hormonal variations.