Hormone secretion by short and long term explant cultures from needle aspiration biopsies of human pituitary adenomas was measured. The results were compared with similarly studied specimens obtained by transsphenoidal hypophysectomy and related to tumor immunocytochemistry and the clinical presentation. There was a high degree of correlation between the pattern of hormone secretion from the pituitary biopsies, the hormones found in the tissue, and the clinical presentation of the patient. In the five aspiration biopsies studied, evidence was obtained that two adenomas secreted solely PRL, one secreted only GH, and two secreted a mixture (one with a preponderance of GH over PRL, and one with a preponderance of PRL over GH). In the latter, the GH and PRL were secreted by separate cells. The three specimens from transsphenoidal hypophysectomies secreted PRL, GH, LH, FSH, and TSH and contained both neoplastic and normal pituitary tissue. In long term culture, GH, LH, FSH, and TSH secretion declined rapidly. PRL secretion from prolactinomas initially remained constant or, in a few cases, increased, but after 20 days again declined, although PRL remained detectable for up to 110 days. It is concluded that short term culture of material obtained from the center of a pituitary by biopsy, but not necessarily specimens obtained by transsphenoidal hypophysectomy, can provide definitive information regarding secretory activity of the adenomas.