Male and female rats that were handled from birth to 30 days had increased adrenal weight at weaning. The animals were then rehoused in (a) groups of 10 in a large cage with an enriched environment, (b) a small cage in pairs, or (c) a small cage alone. Isolation advanced and enrichment retarded vaginal opening (in comparison with paired animals) in handled and unhandled females. Puberty was accelerated by handling irrespective of housing conditions. Isolation improved sexual performance in male rats: almost all ejaculated within 4 daily testing sessions with a receptive female, whereas only half the pair-housed males and even fewer group-housed animals copulated. The group-housed copulators were sexually the most active in a hierarchy observed when a single receptive female was placed in the enriched cage for 15 min. The rank order in which these males initiated and consummated sexual activity was consistent over several tests.