The cumulus oophorus, unique to the egg of eutherian mammals, has been suggested to function either in egg transport to the oviduct ampulla, in avoidance of polyspermy, in enhancement of sperm fertilizing ability, or in sperm guidance to the egg. To examine its role further, the content of the oviduct ampulla was collected at 0700-0730 h, 1130-1200 h, or 1600-1700 h on the day of ovulation from rats mated once or more than once, and analysed with an emphasis on sperm distribution in the ampullary fluid, in the cumulus, and in the eggs. The distribution of spermatozoa between cumulus and ampullary fluid was highly skewed (ca. 9:1) at all times in favor of the cumulus. In 14.1% of the ampullae collected by 0730 h the only spermatozoa present were those already within eggs; and in many other ampullae then there were no more than one or so additional spermatozoa outside fertilized eggs--a testament to the efficiency of the mechanisms that ensure an early meeting of gametes in the oviduct. There was a gradual increase in the total number of spermatozoa in the ampulla throughout the day of ovulation, and the time of eventual cumulus dispersal later in the day generally reflected the number of spermatozoa within it, rather than the occurrence of fertilization per se. These observations suggest that the cumulus oophorus sequesters and brings to the immediate vicinity of the eggs the few spermatozoa which gain the ampulla at or soon after ovulation, so favoring early fertilization.(ABSTRACT TRUNCATED AT 250 WORDS)