Mature Merino ewes (n = 320) were fed either maintenance (100% M) or 25 % maintenance (25% M) rations from the day after mating until embryos were collected at either day 11 or 21 after mating.On the 4th day after mating 223 ewes were ovariectomized and given 5, 10, 15, 20 or 25 mg progesterone/day (intramuscular) until embryo collection. A further group of sham operated ewes (n = 54) acted as controls. Despite live-weight losses of 10-7 and 15-8% from mating to days 11 and 21 after mating respectively, ewes on 25% M rations showed no evidence that embryonic survival was influenced by undernutrition. The embryos from ewes fed 25 % M rations were significantly (P < 0-05) smaller than those fed 100% M (crown-rump measurements were 5-93 0139 mm and 6-39 0-116 mm at day 21 after mating in the 25 % M and 100% M groups respectively). Development, assessed by formation of the first pharangeal lobe and the fore- and hind-limb buds, was significantly retarded (P < 005) in the 21-day-old embryos from the undernourished ewes. Embryo survival in ovariectomized ewes receiving 5 or 10 mg progesterone/day was significantly (P < 0-05) lower than in ovariectomized ewes receiving 20 and 25 mg progesterone/ day at day 11 after mating. Embryo survival at both days 11 and 21 after mating increased with increasing doses of progesterone. Ewes fed 25% M rations had elevated (P < 0-001) concentrations of peripheral progesterone compared with those fed 100% M. This occurred when the progesterone was derived from either exogenous or endogenous (ovarian) sources, dismissing the possibility that changes in secretion rates could account for these observed differences. Differences in embryo growth and development were not related to these variations in the plasma progesterone concentration. Undernutrition restricted both growth and development to 21 days of pregnancy.