The localization of cathepsin B, a potential promoter of prolactin (PRL) release via extra renal renin-angiotensin system in the adenohypophysis, and its inhibitor cystatin C in the human adenohypophysis and in pituitary adenomas were examined using single and dual immunohistochemical staining. In the adenohypophysis, cathepsin B was expressed in about 50% of the ACTH-producing cells, and in 5% or less of the GH- and PRL-producing cells. In contrast, cystatin C was expressed in about 70% of the GH- and PRL-producing cells, but in only 5% or less of the ACTH-producing cells. PRL-producing adenomas strongly expressed cathepsin B, but only weakly expressed cystatin C, a pattern of staining contrary to the expression in normal PRL-producing cells. The above results suggest that cathepsin B may play a role in promoting PRL release especially in PRL-producing adenomas.