When fresh epididymal spermatozoa were cold shocked for 10 or 30 min, then warmed to 24 degrees C, sperm motility was normal, but cold shocking ejaculated or capacitated spermatozoa caused a significant decrease in the percentage of motile spermatozoa and, for capacitated spermatozoa, in the rate of motility. The acrosomes of motile fresh epididymal and ejaculated spermatozoa became crenulated after cold shock, and the percentage of spermatozoa with crenulated acrosomes increased with longer periods of cold shock and was higher when spermatozoa were cold shocked in serum than in saline. When epididymal spermatozoa were cold shocked after incubation for 4 h at 37 degrees C, the acrosomes on spermatozoa which had not undergone an acrosome reaction became swollen and elevated instead crenulated. Epididymal spermatozoa which were cold shocked and then incubated for 4 h at 37 degrees C exhibited acrosome reactions and activation of motility, but had reduced fertilizing capacity when tested in vitro. Spermatozoa incubated in serum and cold shocked were able to penetrate zone-free ova even though their tails had been bent through 180 degrees. It is suggested that cold shock decreases the fertilizing capacity of hamster spermatozoa by interfering with the ability of spermatozoa to bind to and/or penetrate the zone pellucida.