At the beginning of the autumn breeding season, six Rambouillet ewes were treated with intravaginal pessaries containing 20 mg flurogestone acetate on day 10 (oestrus = day 0) of an oestrous cycle. Sponges were removed after 12 days. Five additional ewes served as untreated controls. Vasectomized rams were used twice daily to detect oestrus and blood samples were collected daily from day 1 until 3 days following the post-treatment oestrus. Animals were slaughtered and reproductive tracts were recovered 3 days after the oestrus followingpessary removal. Epithelial, endometrial and glandular measurements were recorded from sections of oviduct, uterus and cervix using a refrigerated microtome. Serum progesterone was quantified by radioimmunoassay. Intravaginal progestagen treatment prevented ewes from exhibiting oestrus during the period in which pessaries were in place. Oestrus was observed, however, in all treated animals within 2 days of sponge removal, resulting in a mean cycle length (±.e.) of 23·2 (±0·2) days compared with 17·6 (±0·2) days (P<0·01) for controls. Serum progesterone profiles in treated and control animals indicated that corpora lutea regression progressed normally in ewes receiving the exogenous progestagen. Neither reproductive tract weight, ovarian weight, ovulation rate nor follicle number differed between groups (P>0·10). Oviductal, uterine, uterine glandular and cervical epithelial cell heights, as well as uterine glandular proliferation and endometrial thickness, were also similar in treated and control ewes (P>0·10). These data suggest that intravaginal flurogestone acetate does not appreciably alter histology of the reproductive tract during the time in which the fertilized ova would enter the uterus. Overt histological changes, therefore, are not likely to be a significant cause of reduced fertility at a synchronized oestrus.