The neurotransmitter serotonin (5-HT) is involved in various physiological functions via multiple receptor subtypes. These have been classified in seven receptor families (5-HT1-7) on the basis of their structures and functional characteristics. In this study, we examined the expression of 5-HT7 receptors in the rat hypothalamic neurons cultured "in vitro" during postnatal development. Neuronal cultures were prepared from postnatal pups P2, P3 and P5, were treated with bFGF and processed by means of immunofluorescence technique using a polyclonal 5-HT7 receptor antibody. In P2, we found a low density of 5-HT7 labeled hypothalamic bFGF-treated neurons and no 5-HT7 immunolabeling in control cultures; in P3, a moderate number of bFGF-treated neurons were observed but they were not bright. No 5-HT7 immunolabeling was found in controls. In P5, a heavy labeling of small sized bipolar neurons was seen in bFGF-treated neurons, while in control cultures, few labeled neurons with a low stained density were observed. These results suggest that 5-HT7 receptors in hypothalamic neurons begin to appear at P5 and then could be involved in many physiological implications that are not exerted at P2 and P3. This indicates that 5-HT7 receptors have a potential significance in the rat hypothalamus during early postnatal development.