Reproduction in the Japanese quail (Coturnix coturnix japonica), as in many other bird species, is controlled by the daily photoperiod. The present experiments have investigated the role of the posterior or tuberal hypothalamus in the neuroendocrine regulation of this photoperiodic response. By using various types of small knives the tuberal hypothalamus was isolated surgically in male quail maintained on short daylengths. The birds were then transferred to long daily photoperiods and after fourteen days the effects on gonadotrophin secretion assessed by measuring testicular growth and the circulating level of luteinizing hormone (LH). Complete deafferentation of the tuberal hypothalamus blocked testicular growth and the plasma level of LH remained low. Partial cuts that severed the afferents entering from anterior and antero-lateral directions also blocked gonadotrophin secretion but cuts severing posterior connections were without effect. Following deafferentation of sexually mature males the LH level fell rapidly to reach a basal concentration within two days. The normal rise in LH that follows the castration of quail on long days was blocked by complete tuberal deafferentation. Electrolytic lesions were placed medially in the tuberal hypothalamus. Those in the postero-dorsal part of the infundibular nuclear complex (p.d.-i.n.c.) completely blocked testicular growth, while those in the ventral portion of the nucleus (v.-i.n.c.) only partially disrupted gonadotrophin secretion.