We have used quantitative in-situ hybridization histochemistry to assess the somatostatin (SRIH) and GH messenger ribonucleic acid (mRNA) content of 81 pituitary adenomas. SRIH mRNA was found in the absence of GH mRNA in only one tumour and in the five cases in which GH and SRIH mRNA were both present, SRIH mRNA prevalence was approximately two orders of magnitude less than GH mRNA. However, no direct correlation between GH mRNA, SRIH mRNA and circulating GH levels was evident at the time of surgery. In contrast, there was no detectable SRIH mRNA probe binding to a thyrotroph adenoma, a gonadotroph adenoma, 29 endocrinologically inactive pituitary adenomas, nine lactotroph adenomas, two first trimester fetal and two normal adult pituitary glands obtained 24 and 36 h post-mortem. These findings corroborate previous reports of SRIH release from somatotroph adenomas in vitro and suggest that paracrine or autocrine mechanisms linking SRIH and GH mRNA operate within normal pituitary tissue. As somatotroph adenomas are thought to be predominantly clonal in origin, the data further suggest that GH and SRIH are synthesized in the same or closely related cells.