Brain-derived neurotrophic factor (BDNF) is involved as an autocrine factor in the regulation of the secretory activity of the neuroendocrine pituitary melanotrope cells of Xenopus laevis. We studied the subcellular distribution of BDNF in Xenopus melanotropes using a combination of high-pressure freezing, cryosubstitution and immunoelectron microscopy. Presence of BDNF, pro-opiomelanocortin (POMC) and alpha-melanophore-stimulating hormone (alphaMSH) within melanotrope secretory granules was studied by triple-labelling immunoelectron microscopy. In addition, intracellular processing of BDNF was investigated by quantifying the number of immunogold particles in different stages of secretory granule maturation, in animals adapted to black or white background light conditions. The high-pressure freezing technique provides excellent preservation of both cellular ultrastructure and antigenicity. BDNF coexists with POMC and alphaMSH within secretory granules. BDNF-immunoreactivity increases along the secretory granule maturation axis (i.e. from electron-dense, via moderately electron-dense, to electron-lucent secretory granules). Immature, low immunoreactive, electron-dense secretory granules are assumed to contain mainly or even exclusively proBDNF. Strongly immunoreactive electron-lucent secretory granules represent the mature granule stage in which proBDNF has been processed to mature BDNF. Furthermore, in moderately electron-dense secretory granules, immunoreactivity is markedly (+79%) higher in black-adapted than in white-adapted animals, indicating that stimulation of melanotrope cell activity by the black background condition speeds up processing of BDNF from its precursor in this granule stage. It is concluded that, in the Xenopus melanotrope, BDNF biosynthesis and processing occur along the secretory granule maturation axis, together with that of POMC-derived alphaMSH, and that the environmental light condition not only controls the biosynthesis and secretion of BDNF and of POMC end-products, but also regulates the rate of their intragranular processing.