Pituitary adenomas obtained from 6 patients with acromegaly and 8 patients with chromophobe adenoma were subjected to the tissue culture to correlate in vitro secretion of growth hormone (GH) and prolactin (PRL) to preoperative in vivo endocrine studies and endocrinilogical manifestations. Tissue culture demonstrated that all of the acromegalic tumors secreted large amounts of GH for 2-4 weeks and some of the PRL-secreting chromophobe adenoma without any clinical and endocrinological evidences of GH secretion secreted considerable amounts of GH; however, no recognizable GH secretion was noted in the remaining chromophobe adenomas. PRL was secreted in vitro in large amounts by tumors obtained from patients with hyperprolactinemia with and without acromegaly. It is interesting to note that tumors obtained from 2 patients who showed very slight elevation of plasma PRL of less than 40 ng/ml secreted large amounts of PRL in vitro. This finding suggests that PRL-secreting adenoma could not be ruled out in patients with mild hyperprolactinemia. The duration of in vitro PRL release was longer than that of GH and its rate of decrease was slower than that of GH.