A total of 339 spontaneous pituitary tumours from 284 aged rats were studied by light microscopy. Based on the cellular morphology in preparations stained by haematoxylin and eosin, tumour cells were classified into 4 types: granular; agranular small; agranular large polygonal; and agranular large slender cells. Eight tumours consisted of mixed neoplastic masses of 2 types of cells. A total of 85 cases representative for each tumour type was used for special staining and immunocytochemistry. All tumours of the granular cell type and some tumours of the agranular large polygonal cell type had granules which stained red by azocarmine and were positive for prolactin. All agranular small cell type tumours were also positive for prolactin and agranular slender cell type tumours were positive for adrenocorticotropic hormone. Only tumours of the agranular large polygonal cell type gave an indefinite or negative reaction for all the various pituitary hormones. These results indicate a relatively good correlation between the cellular morphology and the immunocytochemistry of spontaneous pituitary tumours in aged rats.