This trial sought to increase the knowledge of the changes in the endocrine status of male sheep as they mature, and to link this with sexual and agonistic behaviour when they are reared in the presence or absence of females from weaning.

80 twin-reared Suffolk x Mule lambs were selected at birth. 32 of the male lambs were castrated (C) and 32 were left entire (E); there were 16 female lambs (F). The ewes were stocked at a rate of 20 ewes per hectare. The lambs were weaned at 12 weeks of age in late June, when they were separated into two groups. The first group (+F) comprised 16 entire males, 16 castrated males and 16 females that were run together until the end of July. Thereafter, raddled vasectomized rams were run with the ewe lambs to show when the first oestrous cycles occurred.