Various types of constant-release melatonin implants were designed and administered to adult ewes during seasonal anoestrus. Implants were given between May and July in a series of four experiments. Treatment of ewes with melatonin implants resulted in advancement of the onset of oestrous activity. Melatonin treatment also resulted in improved reproductive performance, increased numbers of lambs born per ewe bred and caused more multiple births. Results from this study show, therefore, that constant-release melatonin implants administered during late spring or early summer advance the onset of the breeding season and improve reproductive performance in ewes.