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Seasonal variation in the incidence and duration of behavioural oestrus in spayed ewes treated with progesterone and oestrogen has been attributed to changes in the sensitivity of ewes to oestrogen (Reardon & Robinson, 1961; Fletcher & Lindsay, 1971). This interpretation is open to question, however, in view of suggestions from Lamond & Bindon (1962) and Lamond (1964) that intact ewes showed seasonal changes in sensitivity to progesterone. The investigation reported here was designed to determine whether changes in sensitivity to progesterone contributed to seasonal variation in the expression of oestrous behaviour in spayed ewes.

The experiment was of factorial design with three different doses of progesterone for priming, two breeds of ewe and two seasons of observation. Sixty spayed Merino and 60 spayed Border Leicester x Merino cross-bred ewes were each allotted at random to three groups of 20. They were injected daily with 5, 10 or 20 mg progesterone