The pars distalis of the tortoise, Geoclemis reevesii, was studied immunocytochemically with regards to its fine structure and distribution of different cell types. The pars distalis consists of cephalic and caudal lobes and a zona tuberalis, each with distinct cellular components. The cephalic lobe contains ACTH cells and prolactin (PRL) cells, while the caudal lobe consists of STH (GH) cells, TSH cells and FSH/LH (GTH) cells. The zona tuberalis contains GTH cells and a small number of TSH cells. ACTH cells, found exclusively in the cephalic lobe, are amphophilic cells containing ACTH immunoreactive secretory granules 200-240 nm in diameter. PRL cells occur only in the cephalic lobe and are large oval or columnar cells containing large secretory granules 300-500 nm in diameter. The cells immunoreactive to anti-chicken LH serum are also reactive to anti-chicken FSH serum, and were identified as GTH cells. They are distributed in the caudal lobe and zona tuberalis, with a few in the pars tuberalis. The GTH cells contain numerous secretory granules of varying density and diameter 200-280 nm, and large globules. TSH cells are small rounded cells containing small secretory granules 180-240 nm in diameter, and distributed in the caudal lobe and zona tuberalis. After thyroidectomy, these cells are hypertrophied and develop to thyroidectomy cells containing large vacuoles and a few secretory granules. GH cells are typical orangeophiles distributed in the caudal lobe. They contain secretory granules 200-400 nm in diameter. The zona tuberalis, which covers the antero-lateral portion of the cephalic lobe, is closely similar in cellular constitution to that of caudal lobe. It may presumably be a structure extended from the caudal lobe and not a homologue of the mammalian zona tuberalis.