Developmental profiles of [3H]mesulergine-labelled central 5-HT1c binding sites differed regionally in the rat during the first 5 weeks postnatal. At birth, cortical 5-HT1c sites were 24-32% of adult levels and increased significantly by 14-21 days. In brainstem, the majority of 5-HT1c sites were already present at birth and increased further by 10 days, which was earlier than in other regions. A pattern of increasing binding site density was also found in hippocampus, diencephalon, and spinal cord. Cerebellar 5-HT1c sites remained at low levels throughout. A mature pattern of regional differences was apparent by 21-28 days with a new rank order: cortex, diencephalon > spinal cord, brainstem, hippocampus > cerebellum. These trends were seen whether Bmax was expressed according to brain weight or protein concentration. The regional contrast in the ontogeny of 5-HT1c binding sites may be relevant to the proposed role of 5-HT as a growth factor in brain development.