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Spermatozoa from the cauda epididymidis of adult male rats were preincubated in different media for various times and used for fertilization in vitro. When examined at different times after insemination, sperm penetration occurred 5 to 6 hr after insemination with epididymal spermatozoa preincubated in a standard medium as used for fertilization, and a marked decrease of penetration rates was observed if the preincubation period exceeded 3 hr. Addition of rat serum to the standard medium did not improve the time and rate of sperm penetration. By contrast, preincubation of spermatozoa in the presence of 2 mMdibutyryl cyclic 3\m='\,5\m='\-adenosinemonophosphate (dCAMP) caused earlier sperm penetration although the overall penetration rates decreased as the time of preincubation increased. Preincubation of spermatozoa in a medium with a high potassium/sodium ratio (0\m=.\32)increased the penetration rates and caused early sperm penetration, while in a medium containing high K/Na ratio and 2 mM-dCAMP the latency of sperm penetration was shortened distinctively and approximately 90% of eggs were penetrated within 3 hr after insemination with spermatozoa preincubated for 5 to 6 hr. It is considered that rat epididymal spermatozoa can be capacitated in a chemically defined medium and that addition of potassium and/or dCAMP accelerates the process of capacitation.

CAPACITATION OF EPIDIDYMAL SPERMATOZOA IN A MEDIUM WITH HIGH K/Na RATIO AND CYCLIC AMP FOR THE FERTILIZATION OF RAT EGGS IN VITRO