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In male sheep, reproductive activity is affected by a range of external factors, including socio–sexual cues, photoperiod and nutrition (Fig. 1). In sexually mature Merino rams, nutrition is arguably the most powerful of these factors and the responses to it can be divided into short-term effects that act mainly on the neuroendocrine system controlling testicular activity (Martin et al., 1994a) and long-term effects that act on testicular growth and sperm production (Oldham et al., 1978). This review will address the nature of the link between nutrition and reproductive function at both testicular and brain levels, demonstrating the complexity of the metabolic inputs and illustrating how far we have been drawn from hypotheses based on single metabolites. The review will conclude with an overview of our working hypothesis of the regulatory mechanism. The main focus will be on our primary experimental model, the mature male sheep, but arguments will be extended to studies in female sheep and other species in which the nutrition–reproduction link has been investigated.