Zona-free eggs were introduced to fresh or preincubated sperm suspensions and the penetration of eggs by foreign spermatozoa was examined, as evidenced by enlargement of the sperm head and formation of the male pronucleus. It was found that zona-free hamster eggs can be penetrated by guinea-pig, deer mouse and rabbit spermatozoa but zona-free rat, mouse and rabbit eggs cannot be penetrated by guinea-pig spermatozoa. Furthermore, zona-free rat and mouse eggs cannot be penetrated by spermatozoa from two species of deer mice and the Mongolian gerbil. The zona pellucida of a few intact rat eggs can be penetrated by mouse (6%) and by P. leucopus spermatozoa (14%) but enlargement of the sperm head and formation of pronuclei were observed in the former but not in the latter. It seems that (1) sperm capacitation is required for the penetration of zona-free eggs, (2) the attachment of foreign spermatozoa to eggs may indicate their potential ability of penetration in some cases, (3) there is a certain affinity between the vitellus of one species and spermatozoa from another species, (4) the block to the entry of foreign spermatozoa is not only in the zona pellucida but also in the vitelline membrane, (5) zona-free hamster eggs can be penetrated by spermatozoa of six species, (6) mouse spermatozoa can penetrate zona-free eggs of three species, and (7) fertilization of intact P. maniculatus eggs can be achieved in vitro.