The ontogeny of somatostatin binding sites was studied in eight fiber tracts of the human lower brainstem in 18 fetuses and infants aged from 21 weeks postconceptional to 6 months postnatal, and in two adults. The study was performed by means of quantitative autoradiography using [125I-Tyr0,DTrp8]somatostatin-14 as a radioligand. For all structures examined, the highest densities of binding sites were detected in the younger stages and the density of sites decreased during development. These results reveal the existence of a close inverse relationship between the density of somatostatin receptors and the myelination process in the fiber tracts of the human brainstem.