Most investigators studying oestrous behaviour in sheep use the incidence of such behaviour as a quantal response, an animal being scored as showing or as not showing behavioural oestrus. This procedure has been useful in studying the endocrinology of oestrus in this species but has not clarified the effects of the ovarian hormones on certain quantitative aspects of oestrus such as its duration. In only one study (Lindsay, 1966) have quantitative measurements of the duration of oestrous behaviour in the ovariectomized ewe been recorded and these results suggest that the length of the induced oestrus might be related to the dose of oestradiol benzoate. Fifty Border Leicester × Merino ewes, ovariectomized 2 years previously, were randomly divided into five groups. The ewes were then injected with 20 mg progesterone every second day for 10 days, followed by a final injection of 10 mg progesterone 2 days later. Forty-eight hours after