Karakul, Mehraban, Naeini and Bakhtiari sheep were checked twice a day for oestrus using aproned rams and were laparotomized within 5 to 16 h of the end of every second oestrous period. Oestrus was observed for 12 monthly periods commencing on 21 April.
The Mehraban breed with 1 -6 oestrous periods per month was less seasonal in its breeding activity (P<0·01) than the other breeds. Four-year-old ewes were more sexually active (P<0·01) than 3-year-old ewes (1·48 v. 1·28 oestrous periods per month). Seasonal variation in the sexual activity of the ewes was observed (P<001); the peak of activity in July to January was followed by a steady decline in February and March to a low in April and May, and then a steep recovery phase in June. There was only limited sexual activity for at least 4 months (March to June).
The overall mean cycle length was 17·8 days (14 to 23 days), and the means for the four breeds were 17·6, 17·8, 17·8 and 17·9. The oestrous cycle was longer (P<0·05) during the period from December t o February than during that from June to August.
The overall mean duration of the oestrous period was 36·9 h and the means for the four breeds were 35·2, 38·0, 37·4 and 37·1 h, respectively (P<0·05). The duration showed some monthly variation (P<0·01) but did not follow a special seasonal pattern. The ovulation rates for the four breeds were 1·07, 1·13,0·94 and 0·95 respectively, (P<0·01). There was a non-significant decline in the mean ovulation rate from March to June. Right ovaries were more active than left ovaries as judged by the observation of a higher average number of corpora lutea per ewe per laparotomy (0·58 v. 0·43, P<001).