We have assessed, in post-partum Merino ewes (n = 120) lambing during the ovulatory season, the influence of nutrient status on the onset and cessation of ovarian cyclicity and on the oestrous and ovulatory responses to ram introduction. Two flocks were studied from mid-pregnancy onwards; flock R + + included vasectomized rams (8%) and flock R - + was isolated from rams until Day 1 of the study when the ewes were 21-37 days post partum. Within each flock, from mid-pregnancy until Day 7, three groups of ewes (n = 20/group) received normal or restricted nutrition, or restricted nutrition with a supplement of lupins and oats from Day - 14 to Day 7. After Day 7 all ewes received similar normal nutrition. All ewes received progestagen from a pessary from Day - 7 to Day 1. On Day 1 the rams were removed from flock R + + and new vasectomized rams (10%) were introduced into both flocks. Compared with ewes of normal nutrient status, the ewes of reduced nutrient status had a longer (P<0.05) interval to first oestrus (65.1±3.4 vs 37.7±3.2 days, mean±s.e.m.), an earlier (P<0.05) cessation of oestrous cycles as assessed from the time of the last observed oestrus within the period of study (to Day 115) (110.5±5.6 vs 128.5±4.0 days) (flock R + +) and a lower (P<0.001) incidence of oestrus (4/20 vs 14/20) within 14 days of ram introduction, but a similar (P>0.05) incidence of ovulation (as assessed from plasma progesterone on Day 13) (15/19 vs 19/20) (flock R - +). Short-term nutrient supplementation did not affect these responses. The occurrence of oestrus within 14 days of ram introduction was similar (P>0.05) in the ewes receiving normal nutrition in flock R - + and flock R + + (14/20, 14/20). These results demonstrate that reduced nutrient status of post-partum ewes can (a) delay the onset of ovarian cyclicity post partum; (b) inhibit the occurrence of ram-induced oestrus (but not ovulation); and (c) hasten the onset of cessation of oestrous cycles.