The influence of estrous cycle and intrastriatal implants of 17 beta-estradiol (17 beta-estradiol). 17 alpha-estradiol (17 alpha-estradiol) or cholesterol on the number of footfaults made by female rats traversing a narrow suspended beam was investigated. Female rats made fewer footfaults on estrus than on other days of the cycle. This was true when testing occurred during either the light or dark phase of the light:dark cycle. Intrastriatal implants of 30% 17 beta-estradiol for 6 hours resulted in a significant improvement in sensorimotor performance as soon as 4 hours after hormone implant and persisting for days. In contrast, intrastriatal implants of either 30% 17 alpha-estradiol or cholesterol had no influence on performance. The extent of hormone diffusion away from the implant cannula was minimal, and the resulting concentration of 17 beta-estradiol in the striatum was less than 10 pg/mg. It is concluded that estradiol has a direct, stereospecific effect in the striatum that influences performance of a skilled motor act in the female rat.