The threshold and pattern of electroshock seizures of 100 mature rats during the estrous cycle were investigated. It was observed a) that the threshold for minimal seizures was highest during diestrus, lower during proestrus, and lowest during estrus; and b) that the duration of tonic flexion during maximal seizures was longest during diestrus and shortest during estrus. These findings indicate that convulsive reactivity in the rat is lowest during diestrus, higher during proestrus, and highest during estrus. This heightened reactivity during estrus may be the expression of an increased central excitability as a result of high levels of circulating estradiol during this phase. It was also observed that electroshock thresholds were significantly lower between 9:00 and 11:00 pm than between 1:00 and 3:00 pm when illumination was from 7:00 am to 7:00 pm. These day-night differences or circadian rhythms in thresholds were quantitatively larger than changes seen during the estrous cycle. Finally, convulsions produced by these methods did not appear to alter the regularity of the estrous cycle.