Guinea pig eggs can be fertilized in vitro in a chemically defined medium originally developed by Biggers, Whitten and Whittingham for the cultivation of fertilized mouse eggs. When unfertilized eggs were inseminated in this medium (abbr. BWW) with fresh epididymal spermatozoa, sperm penetration occurred not earlier than two hours after insemination. Quick and efficient sperm penetration occurred when the eggs were inseminated with spermatozoa pre-incubated in BWW for 11-18 hours. Spermatozoa started to pass through the zona pellucida as early as 10-15 minutes following insemination and all the eggs were penetrated by spermatozoa within one hour. The time needed for functional capacitation of guinea pig spermatozoa is estimated to be no less than two hours under the experimental conditions used, the optimal time possibly being 8-12 hours or even longer. The possibility of in vitro sperm capacitation and fertilization of eggs in media much simpler than BWW (e.g., Earle's solution with or without albumin) is suggested.