The spontaneous maturation of intact, cumulus-free and zona-free oocytes of the mouse was studied in complex and in simple media. The rates and frequencies of maturation from the germinal vesicle to the metaphase II stage were similar for all oocytes, indicating that these investments are not critical to maturation. The penetration characteristics of zona-free oocytes were examined; before germinal vesicle breakdown (GVB), evidence for penetration was obtained only when the ionophor, A23187, was included in the medium. After GVB, oocytes readily incorporated spermatozoa and were usually polyspermic when inseminated with 10(5) capacitated epididymal spermatozoa/ml. The kinetics of sperm incorporation indicated that primary oocytes were capable of a plasmalemma block to penetration similar to that seen with tubal eggs.