Circulating levels of melatonin following its oral administration or subcutaneous injection in sheep and goats.

The effect of differing doses and routes of administration of melatonin on plasma melatonin levels in sheep and goats has been examined. Melatonin injected subcutaneously in a saline or oil vehicle caused high transitory peaks in plasma melatonin whereas oral administration, in either saline solution or adsorbed onto pelleted foodstuff, resulted in sustained elevated blood levels for periods exceeding 7 h. Oral dosages of about 2 mg proved adequate to raise the normal daytime plasma levels in both sheep and goats to levels within the normal night-time range. It was concluded that with ruminants the oral route of administration provides a convenient and practical way of administering melatonin for physiological study.