It has been shown that giving mule ewes a soluble glass bolus which contained 150 rag melatonin on 2 July, advanced the onset of the oestrus cycle. This experiment was designed to investigate whether the dose rate of melatonin could be reduced.

• 1.40 ewes not treated (control).

• 2.40 ewes given glass bolus containing 150 mg melatonin.

• 3.40 ewes given glass bolus containing 100 mg melatonin.

120 mule ewes (Blue-faced Leicester x Swaledale) aged between 4 and 6 years all of proven breeding ability were divided into 3 similar groups by age, weight and body condition on 2 July. One group was left untreated and 2 groups received a polymer-bonded soluble phosphate glass bolus (Pilkingtons P.L.C.) which contained the appropriate level of melatonin (Sigma Chemicals Corporation). Each bolus was cylyndrical in shape measuring 1.8 cm in diameter and 3.0 cm in length. Tho average mass and specific gravity of boluses were 23 g and and 3.0 respectively.