A single oral dose of mestranol (l7ct-ethynylestradiol 3-methyl ether) was administered to two groups of eight adult female beagle dogs each on Day 6 or 21 after first acceptance of the male. Based on corpora lutea counts, embryonic loss was 95.5 and 67.3%, respectively, for the two treatment groups, and only 34.5% for the control. In a second study a group of 12 beagles were used to determine the effect of mestranol on ova transport. Nine bitches were dosed on Day 6 and the reproductive tracts were removed between 1 and 5 days later and three controls were examined on Day 11. Thirty-nine of a possible 56 ova were flushed from the tracts of the treated females and all but one were from the oviducts. Eighteen (46%) of the ova were degenerating. The controls produced 17 normal zygotes out of a possible 24, and all were flushed from the uteri. Mestranol apparently did not accelerate ova transport, and the possibility of both a direct effect on the ova and an indirect effect via a delay in ova transport is discussed.