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The amphibian Xenopus laevis is able to adapt to a dark background by releasing melanophore-stimulating hormone from the pars intermedia of the pituitary gland. The inhibition of melanophore-stimulating hormone release is accomplished by neuropeptide Y-containing axons innervating the pars intermedia. To determine the production site of neuropeptide Y involved in this inhibitory control, the distribution of neuropeptide Y in the brain has been investigated by immunocytochemistry and in situ hybridization. Immunoreactive cell bodies were visualized in, among others, the ventromedial and posterior thalamic nuclei, and the suprachiasmatic and ventral infundibular hypothalamic nuclei. A positive hybridization signal with a Xenopus-specific probe for preproneuropeptide Y-RNA was found in the diencephalic ventromedial thalamic nucleus and in the suprachiasmatic nucleus. With both immunocytochemistry and in situ hybridization, suprachiasmatic neurons appeared to be stained only in animals adapted to a white background; animals adapted to a black background showed no staining. Quantitative image analysis revealed that this effect of background adaptation is specific for suprachiasmatic neurons because no effect could be demonstrated of the background light condition on the ventral infundibular nucleus (immunocytochemistry) or the ventromedial thalamic nucleus (in situ hybridization). These results indicate that neurons in the suprachiasmatic nucleus enable the adaptation of X. laevis to a white background, by producing and releasing neuropeptide Y that inhibits the release of melanophore-stimulating hormone from the melanotrope cells in the pars intermedia of the pituitary gland.