Few human pituitary tumor cell lines have been established. This scarcity may be related to the fact that hormone production rapidly declines within weeks to months after initiation of the culture. In addition, the number of proliferating cells in pituitary adenomas is very low. The use of primary cultures of human pituitary adenomas to study the mechanisms underlying the regulation of hormone synthesis and release is limited by the small quantity of tumor tissue which can be obtained by transsphenoidal surgery. The use of primary cultures yields considerable information regarding the sensitivity of hormone secretion by pituitary adenoma cells to drugs like somatostatin analogs and dopamine agonists as well as to various stimulatory and inhibitory neuropeptides. The establishment of growing tumors in nude mice transplanted with human pituitary adenomas has also been shown to be difficult and of limited success. Therefore, the study of human pituitary gland tumor cell function is limited mainly to primary cultures of human pituitary adenomas.