Urea-supplemented wheat straw (UWS) was substituted stepwise by urea-ammoniated wheat straw (AWS) in diets fed to SA Mutton Merino ewes during late pregnancy and lactation. The experimental animals were fed ad libitum under zero grazing conditions over the last 8 weeks of pregnancy, and from lambing to weaning at 67 ± 8 days. The ratio of UWS to AWS was 60:0, 30:30, and 0:60 in the respective late pregnancy diets and 50:0, 25:25, and 0:50 in the corresponding lactation diets. The composition of the concentrate in the late pregnancy diets and in the lactation diets was similar. The in vitro organic matter digestibility (IVOMD) of diets, arranged according to increasing AWS content, was 50,2; 54,8 and 57,5% for the late pregnancy diets, and 61,3; 63,6 and 66,8% respectively for the lactation diets. The daily voluntary dry matter intake of all the diets was lower than expected over the entire experimental period, and unaffected by AWS inclusion. The body mass of the ewes was relatively constant on all the diets during the last 6 weeks of pregnancy. The body mass of ewes on the lactation diet containing only UWS was significantly (P < 0,05) lower than in the AWS-containing diets during the period from 4 to 8 weeks after lambing. The plasma glucose levels of the ewes were rather low over the entire experimental period and unaffected by AWS inclusion. The birth mass of the lambs on the diet containing only AWS was approximately 10% higher than that of the lambs on the other diets (P = 0,06). The ammoniation of wheat straw had no advantage in terms of lamb growth and survival over supplementation with urea.