Thirty-eight does were double mated to two bucks whose offspring were distinguishable from each other. The interval between mating to the first buck and mating to the second buck was ½ 1 or 2 hr. The first buck was mated 10, 9, 8, 7, 6, 5 or 4 hr before ovulation. The first buck sired 86% of the offspring when the interval between bucks was 2 hr. When the interval between males was shortened, the first male sired the majority of offspring but the proportion was less than that after a 2-hr interval. It was concluded that spermatozoa that have resided in the genital tract of the doe have a competitive advantage over other spermatozoa that have spent less time in the female. The advantage did not disappear after the 6 hr residence, usually thought to be adequate for capacitation, but extended to at least 10 hr. Capacitation (maturation) may possibly be a quantitative phenomenon as well as a qualitative one.