The rapid effect of 17beta-estradiol upon filopodial growth was investigated using videomicroscopy of hippocampal neurons in culture. Within 1-10 min, 17beta-estradiol induced a significant increase in the number of filopodia decorating neuritic extensions. In addition, the length of existing as well as newly formed filopodia increased significantly during the 10-min 17beta-estradiol exposure. Neurite area and neurite length did not change significantly. The inactive epimer, 17alpha-estradiol, had no effect upon any of the morphological parameters assessed. Similarly, neither progesterone or corticosterone showed a significant influence upon any of the parameters of hippocampal nerve cell growth in culture within the same time frame in which 17beta-estradiol effects were observed. Testosterone induced a significant increase in the number of filopodia without a significant increase in the overall length of the filopodia. Results of these studies demonstrate that 17beta-estradiol can induce rapid changes in nerve cell morphology, suggesting that modification of neuronal circuitry can occur within minutes of exposure, a time base that is consistent with 17beta-estradiol influence upon nerve cell excitability.