Two hundred and eight Dormer ewes were included in an investigation into the effect on ewe fertility of dihydrostreptomycin antibiotic pessaries used with fluorogestone acetate-impregnated intravaginal (FGA) sponges for the synchronisation of oestrus and ovulation early in the breeding season. In addition, the vaginal environment and flora were investigated. Although synchronisation was good, only 70% of the ewes exhibited oestrus and of these 42% failed to conceive. Maiden ewes had a lower oestrous response (50% cf. 81%), but conceived as well as mature ewes. Although the use of antibiotic pessaries did not improve oestrous response or conception rate, as a result of a higher twinning rate following this treatment, more lambs were born per ewe lambing (1,41 cf. 1,26; P<0,05). It is suggested that the antibiotic pessaries were beneficial to sperm survival in the genetal tract of the ewe.