In primates, as in nonprimates, it is well established that periods of chronic or severe undernutrition can lead to a suppression of reproductive hormone secretion. Recent studies in monkeys and humans indicate that reproductive hormone secretion begins to be suppressed with even very brief periods of undernutrition. Specifically, in male monkeys reproductive hormone secretion responds to missing a single meal, eating a single meal, and to a change in the timing of daily meal intake. These findings suggest that nutritional/metabolic signals that are linked to food intake are part of the normal physiological mechanism regulating the daily activity of the reproductive axis, rather than simply signals that influence the activity of the reproductive axis in pathological conditions of severe undernutrition. The nature of the metabolic cue(s) linking reproductive hormone secretion to subtle changes in the metabolic status of the body remains unknown, as does the route by which metabolic information reaches the central hypothalamic neurones governing reproductive hormone secretion. However, recent studies indicate that the metabolic cue transmitting information to the reproductive axis is dependent on calorie intake, but is not dependent on changes in body mass or composition, changes in intake of a specific nutrient, changes in plasma glucose or insulin concentrations, or signals emanating from the taste or smell of food or the physical process of food ingestion.