The effect of exercising Merino rams on the lambing performance of 2 227 Merino ewes was investigated. The ewes were divided into three groups of which two were mated to exercised rams and the third to non-exercised rams. No significant differences were obtained in body mass between the exercised and non-exercised rams. A significant (P ≤ 0.05) difference in cumulative lambing percentage over an eight week lambing season was found between ewes mated to exercised rams (91.9%) and ewes mated to nonexercised rams (75.9%).