When pregnant adolescent sheep are overnourished to promote maternal growth during pregnancy, growth of the placenta is impaired and results in the premature delivery of low birth weight lambs relative to control-fed adolescents of equivalent age. These effects have been achieved by feeding two levels of the same complete diet. The present study evaluated the role of protein in pregnancy outcome in our adolescent sheep paradigm. Adolescent ewes were implanted with single embryos on day 4 post-oestrus. Thereafter ewes were offered ad libitum an isoenergetic diet (11.4 MJ metabolisable energy/kg DM) containing either 12% (basic, B) or 17% (extra, E) crude protein. At day 75 of gestation, half the pregnant ewes on each protein level were switched to yield four groups, BB, EE, BE and EB protein. A further optimally nourished control group received a moderate quantity of a ration (14% crude protein) designed to provide 100% of the estimated energy and protein requirement of the adolescent sheep according to stage of pregnancy. Pregnancy outcome was determined at term. Feed intakes were independent of protein level in the four groups of ewes fed ad libitum and were higher (P<0.001) than in the control group throughout. Maternal plasma urea concentrations reflected the current crude protein content of the diet offered and were elevated in the 17% compared with 12% protein groups (P<0.001). Within groups fed ad libitum, maternal plasma insulin, glucose, NEFA and homocysteine concentrations were largely independent of protein level. Gestation length, placental weight, lamb birth weight and initial colostrum yield were reduced (P<0.05) in all groups fed ad libitum relative to the optimally nourished control group. Similarly, total colostrum IgG, butterfat, lactose and crude protein content at parturition were attenuated in the ad libitum compared with the control groups. However, within ad libitum groups pregnancy outcome parameters were largely unaffected by level or timing of exposure to high protein intakes. The data imply that it is high-energy intakes that are the primary cause of impaired placental development and adverse pregnancy outcome in rapidly growing adolescent sheep.