Morphological characteristics of TSH-immunoreactive cells (TSH-like cells) in the pars tuberalis (PT) of intact adult male rats were studied by light and electron microscopic immunocytochemistry. TSH-like cells in the rat PT can be classified into three types by light microscopy: 1) cells diffusely stained throughout the cytoplasm, 2) diffusely weak stained cells containing a strongly stained spot in the paranuclear region; 3) cells with a strongly stained spot in the paranuclear region without diffuse reaction. We call the latter two cell types "spot cells". Electron microscopic immunocytochemistry revealed that the latter two cells reacted very strongly in the Golgi apparatus. The spot cells were localized throughout the PT from rostral to caudal extremities as well as the ventral surface area of the pars distalis (PD) which was connected to the PT. The cytoplasm of the TSH-cells in the PD proper was strongly and diffusely stained with anti-TSH serum, while there was no spot-like staining in any cells. The spot cells demonstrated by electron microscopy contained a well developed Golgi apparatus, many microvesicles and a few secretory granules. These immunocytochemical results clearly showed two apparently distinct types of TSH-immunoreactive cells between the PT and PD in the intact adult male rat adenohypophysis. This morphological distinction of TSH-immunoreactive cells suggests functional differences in these cells.