Two types of endocrine-paracrine (EP) cells have been detected histochemically and ultrastructurally in normal and hyperplastic prostates; i.e. type 1 cells resembling intestinal EC (enterochromaffin) cells and type 2 cells similar to urethral EP cells previously reported by Casanova et al. (1974). About one-third of the 40 prostatic carcinomas studied contained EP cells. Four tumours showed a very large number of EP cells: two of these were composite tumours exhibiting both adenocarcinomatous and carcinoid patterns. These four tumours have also been studied histochemically and ultrastructurally. ACTH and β-endorphin immuno-reactive cells, ultrastructurally resembling pituitary corticotrophic cells, have been identified in three tumours. Cells identical with type 1 and type 2 cells of the normal prostate were detected in two cases and in a further case, respectively.