This study was designed to test whether an acute improvement in diet would increase gonadotrophin secretion and testicular growth in strongly photoperiod-responsive Suffolk rams and weakly photoperiod-responsive Merino rams in both the breeding (February-March) and the non-breeding (July-August) seasons. Mature rams (n = 5 or 6) of these breeds were fed a maintenance diet (0.9 kg chaff + 100 g lupin grain) or the same diet supplemented with 1.5 kg lupin grain for 42 days in each season. Lupin grain is a rich source of both energy and protein. Testosterone, luteinizing hormone (LH) and follicle stimulating hormone (FSH) were measured in plasma from blood sampled every 20 min for 24 h on Days -1, 12 and 35 relative to the change in feeding. In rams supplemented with lupins, body mass increased in both breeds in both seasons (P < 0.001). Scrotal circumference and LH pulse frequency increased with lupin supplementation in both seasons (P < 0.003) in Merinos, but only during the breeding season (P < 0.003) in Suffolks. Plasma FSH concentrations were affected by diet only during the breeding season, being elevated on Day 12 in lupin-supplemented rams of both breeds (P < 0.05). It was concluded that Merino rams exhibit reproductive responses to improved nutrition irrespective of time of the year, whereas Suffolk rams respond to nutrition only when the hypothalamic reproductive centres are not inhibited by photoperiod. Thus, Suffolks do respond to nutrition, just as Merinos do, but only when photoperiod allows. This difference between breeds appears to be a result of differences in the neuroendocrine pathways that control pulsatile gonadotrophin-releasing hormone secretion.