Ninety-eight sheep in three farm flocks were treated in July (period of late anoestrus) with intravaginal sponges impregnated with 30 mg ‘Cronolone’ (9α-fluoro-11βhydroxy 17α-acetoxy-progesterone; G. D. Searle) or ‘Cronolone’ plus 400 mg progesterone and incorporating doses of 0–800 μg diethyl stilboestrol. Treatment was for 12 days; a single dose of 500 i.u. PMS (Burroughs Wellcome) was injected at pessary withdrawal. Ten rams bred the ewes at the induced and subsequent heats.

Of the ninety-five ewes retaining pessaries, ninety-three (98%) were bred in the 3 days following treatment. Sixty-three sheep (68·5%) conceived at the controlled oestrus, producing an average of 1·54 lambs/conception. In the thirty-four ewes failing to conceive at that time, twenty-seven (80%) showed oestrus after one cycle interval; twenty of these (74%) conceived and produced 1·3 lambs/conception. Lambing outcome toservices at the controlled oestrus was lowest in sheep treated with 800 μg stilboestrol, but the reduction was not significant.