When hamster epididymal spermatozoa were incubated in vitro in the presence of follicular fluid in Tyrode's solution for 3 hr or more, they became fully capacitated. The majority of the capacitated spermatozoa had lost the acrosome cap, and they displayed an extremely vigorous motility. When placed in contact with freshly ovulated eggs in vitro, the spermatozoa started to enter the zona pellucida of the eggs as early as 10 min after insemination; sperm penetration through the zona pellucida, however, occurred most often between 30 and 50 min after insemination. The follicular fluid markedly reduced its spermcapacitating potency when diluted with a large volume of Tyrode's solution. Cumulus oophorus cells, their matrix, corona radiata cells and the eggs had no potency to induce functional capacitation of the spermatozoa. Follicular fluids of four different species were tested. In respect to their potency to capacitate hamster spermatozoa, the fluids of three species could be arranged in the following order: hamster>mouse >rat. Rabbit follicular fluid was totally ineffective.