Photostimulation is an effective means of manipulating reproductive activity in most British breeds of sheep. Short term light treatment can initiate oestrus cycles during June/July and thus provides the opportunity for autumn lambing (Williams, 1977). The most effective light treatments comprise a period of supplementary light followed by a period of long nights which requires controlled environmental housing. Recent investigations have demonstrated that melatonin treatment may replace the long night phase thus simplifying management (Williams, 1985). Attempts to dispense with the period of supplementary light by transferring Spring lambing Welsh Mountain ewes abruptly to long nights during early Spring were not as successful as the conventional type of light treatment (Williams, 1967). This investigation was undertaken to compare the response of adult Suffolk crossbred ewes given melatonin in the Spring with or without a priming period of supplementary light.