The fine structural changes were studied in the plasma membrane, cortical granules (CGs) and meiotic spindle of rabbit, hamster and mouse eggs in response to electrical stimulation. Eggs were collected 16 to 18 hours after HCG injection, and freed from the cumulus oophorus. They were stimulated in vitro by delivering a single monophasic square wave pulse of 150 V for 1 msec. Stimulated and unstimulated eggs were fixed for fine structural observations 1, 30 and 60 minutes after stimulation. Within one minute of stimulation the microvilli of rabbit eggs were long, branching and had bulbous ends. Umbonate protrusions were also present on their surface. By 30 minutes after stimulation the rabbit eggs lacked microvilli and the perivitelline space was filled with detached vesicles. Degenerating changes were readily noticeable in the microvilli of hamster eggs by 60 minutes. Changes were not noted in the microvilli of stimulated mouse eggs. There were markedly fewer CGs in the hamster and mouse, but not rabbit, eggs by 30 minutes after stimulation, and nearly all of them disappeared in the mouse and hamster eggs by 60 minutes. The density of the CGs in the rabbit was not altered. The meiotic spindle of hamster eggs dissipated within one minute of stimulation, however, the microtubules reappeared by 60 minutes after stimulation. Rotation of the meiotic spindle occurred in the mouse and hamster eggs by 30 minutes after stimulation. In some of the mouse eggs the spindle migrated to the center and the chromosomes were in telophase.