The purpose of this study was to detect in vitro growth hormone (GH) and prolactin (PRL) secretion from adenomas clinically associated with GH or PRL hypersecretion. The reverse hemolytic plaque assay (RHPA) was applied in order to reveal possible differences among various morphologic adenoma types, and to examine the inhibitory effects of octreotide on GH release as well. The 20 surgically resected pituitary adenomas studied included 15 from acromegalic patients and 5 from patients with hyperprolactinemia. All adenomas were diagnosed by histology, immunocytochemistry and electron microscopy. Among tumors associated with acromegaly, 5 were densely granulated (DG), 5 were sparsely granulated (SG) somatotroph (SM) adenomas, 2 were mammosomatotroph (MSM) and 3 mixed somatotroph-lactotroph cell (mixed SM-LT) adenomas; tumors causing hyperprolactinemia included 4 lactotroph (LT) adenomas and 1 mixed SM-LT adenoma. GH release assessed by the RHPA corresponded to in vivo hormone secretion and to tissue immunoreactivity. Statistical analysis showed significant differences among all morphologic types of SM adenomas, exclusive of SG-SM adenomas compared to mixed SM-LT adenomas. The mean plaque size in DG-SM and MSM adenomas was significantly greater than that of SG-SM and mixed SM-LT adenomas, indicating higher GH secretion by the former two types during the same incubation time. PRL secretion was documented in 2 mixed SM-LT adenomas. Plaques for PRL, but not for GH were formed in all LT adenomas. In all SM and LT adenomas, cells producing large plaques represented a minority of the plaque-forming cell population, however, they accounted for the largest part of the total plaque area, thus the largest part of hormone secretion. Octreotide effects on GH release were studied in 6 adenomas by the RHPA. Octreotide treatment induced a rapid and significant reduction in GH secretion by SM cells in vitro, with a selective effect on high-secreting cells.