We examined the expression of 5-hydroxytryptamine(7) (5-HT(7)) receptor protein in developing and adult rats with immunohistochemical technique. In adult male rats, 5-HT(7) receptor immunoreactivity was detected in the septum, striatum, indusium griseum, tenia tecta, thalamus, hippocampus and hypothalamus in the forebrain as well as the pons and cerebellum. In brains of 1, 7, 15 and 21 days old male rats but not of adult ones, 5-HT(7) receptor immunoreactivity-positive dot-like structures were detected. The dot-like structures were visualized in hypothalamus, hippocampus, frontal cortex, brainstem and cerebellum at 1 day old male rats. In 7 days old male rats, the dot-like structures were found in the hypothalamus, medial preoptic area (MPA), bed nucleus of the stria terminalis (BST), amygdaloid nucleus and brainstem reticular formation. In 15 and 21 days old male and female rats, 5-HT(7) receptor immunoreactive dots were most clearly detected in MPA, hypothalamus, raphe pallidus, raphe magnus and brainstem reticular formation. The 5-HT(7) receptor immunoreactivity-positive dot-like structures were shown in the cytoplasm and they were less than 1 microm in diameter in 1 and 7 days old rats and became larger to 1-3 microm in 15 and 21 days old rats. From the distribution and morphologic features, the 5-HT(7) receptor immunoreactivity-positive dot-like structure found in developing rat brains is considered to be identical to a cytoplasmic inclusion named 'stigmoid body'.