Neonatal modification of the microbial inoculum entering the gastrointestinal tract mayalter the rumen microbiome and consequently alter pre- and post-weaning rumen fer-mentation and growth of lambs. This study aimed to determine (1) if modifying the rumenby providing ewes with lipids differing in rumen-availability in late gestation and lacta-tion would affect performance of the lamb offspring and (2) whether cross-inoculation ofneonatal lambs with digesta from sheep on an alternate diet would modify the fermen-tation, size, and characteristics of the developing rumen and performance of lambs. Twodiets were offered ad libitum to 36 pregnant ewes (and to their lambs after lambing) from1 month pre-lambing until 2 weeks after weaning, after which lambs were grazed in treat-ment groups in paddocks. Diets consisted of 92% of a blend of oaten and lucerne choppedhay, 4% molasses and 4% fat (coconut oil ? CO or protected fat ? PF). Newborn lambs wereinoculated weekly while suckling (weeks one to eight postnatal) with fresh rumen fluidfrom donor ewes eating CO or PF diets, or were inoculated with water. Lamb body weight,condition score, wool growth, rumen fermentation, and rumen development were stud-ied during the first 5 months of life of the lambs. Diet and inoculation affected dry matterintake after weaning (P