Photoperiodically induced testicular growth does not occur in Japanese quail (Coturnix coturnix japonica) if the posterior or tuberal hypothalamus is isolated surgically using a small knife. The present studies were designed to locate more precisely the origin of the controlling fibres by placing electrolytic lesions and knife cuts throughout the anterior ventral hypothalamus. Lesions in the preoptic region blocked testicular growth, as did cuts immediately posterior to this region. Deafferentation slightly more posterior in the supraoptic region, or rostral to the preoptic area had no effect on gonadotrophin secretion. Cuts in the mid-tuberal hypothalamus were without effect. The role of the preoptic region in the photoperiodic response is discussed.