ABSTRACT. Early maternal care contributes to the normal development of the physiological, social and emotional functions of the offspring. As sheep are a strongly gregarious species, separation and isolation of an individual from the flock provokes fear and, consequently a stress response. The first aim of this study was to compare the stress response to social isolation of rams that were reared by their mothers or artificially reared. A complementary objective was to compare the cortisol response to an ACTH challenge in both groups of rams. Rams were:1) separated from their dams 24?36 h after birth (Week 0) and artificially fed with sheep milk until Week 10 (group AR, n = 14); or 2) reared by their dams until Week 10 (group DR, n = 13). The rams were then kept in separate, adjacent paddocks for the rest of the study. When they were 8 months old, 10 animals randomly selected from each group were isolated from the group in an unfamiliar pen during 10 min. Behavioural (the number of lines crossed, frequencies of vocalizations, eliminations, sniffing, escape attempts and freezing), physiological (heart and respiratory rate, and rectal temperature), biochemical (total blood proteins, globulins and albumin), and endocrine (serum cortisol concentrations) variables were recorded. Dam-reared rams eliminated (defecations + urinations) more times and crossed more lines during the social isolation test than AR lambs (P