ABSTRACT. The objective of this study was to determine if artificially reared male lambs differ from those reared by their mothers in their reproductive development and sexual behaviour during the first breeding season and in their serum testosterone to a GnRH challenge at the end of the first breeding season. Lambs were assigned to two experimental groups:1) artificially reared lambs, separated from their dams 24-36. h after birth (Week 0) and fed sheep milk until 10. weeks of age (group AR, n = 14); and 2) lambs reared by their dams until 10. weeks of age (group DR, n = 13). Reproductive parameters and sexual behaviour were recorded from Weeks 9 to 39. The GnRH challenge was performed on Week 40. Body weight, scrotal circumference, gonado-somatic index, testosterone concentration and sperm parameters were unaffected by group, but increased with age (P