SUMMARY: Four trials were conducted each with seventy-two ovariectomized Suffolk crossbred ewes, to determine the requirements of oestradiol benzoate (ODB) for behavioural and vaginal oestrus when, preceded by progesterone injections for periods of 3, 6 and 12 days. Effective daily requirements of progesterone were also determined. There was no interaction between the daily dose-level or duration of stimulation by progesterone and the vaginal response to ODB. The respective values of ED50 and ED90 of ODB (in μg) for vaginal smears, with their associated 95% fiducial limits, were 8·4-10·7-12·1 and 16·8-18·6-21·9. There was a progressive increase in sensitivity to ODB, as measured by oestrous behaviour, with increasing duration of progesterone pretreatment at daily dose-levels between 3 and 24 mg. The respective values of ED50 of ODB (in μg) injected following 3, 6 and 12 days' effective pretreatment, with their associated 95% fiducial limits, were 17·0-18·7-20·5, 14·8-15·7-16·6 and 11·9-13·1-14·5. The respective values of ED90 were 24·5-27·0-30·7, 20·9-22·6-25·3 and 17·0-18·9-22·0. The requirement of ODB to induce behavioural oestrus in 90% of ewes, after pretreatment with progesterone for 12 days, was indistinguishable from that needed for characteristic vaginal changes. This clearly suggests a close approximation to physiological time-dose relationships. The effective daily dose of progesterone, for maximum oestrous response, lay between the limits of 3 and 12 mg administered as a divided dose. A daily dose of 24 mg appeared slightly less effective, while 1·5 mg/day was much less effective.