Endometrial biopsy specimens (4 or 5 on each occasion) were collected from 7 mares 2, 3, or 4 times over a 50-day period prior to breeding. Four of the collection days were within 6 days of breeding. Six of the 7 mares were diagnosed as pregnant by use of ultrasonography at day 14 after ovulation. This pregnancy rate was the same as that achieved by these mares when they were bred at estrus before the start of the study. It appeared that repeated collection of multiple endometrial biopsy specimens from genitally normal mares did not adversely affect pregnancy rate.