Several earlier studies have shown that some human pituitary GH-secreting somatotrophinomas are able to synthesise and release hypothalamic GHRH and it has been proposed that a positive autocrine feedback loop involving this tumor-derived GHRH may participate in tumorigenesis. We have used in-vitro cell culture and exploited an antagonist to GHRH, (Ac-Tyr1,D-Arg2)-GHRH (1–29)-amide (GHRH-A), to further investigate whether an autocrine loop involving somatotrophinoma-derived GHRH may exist. In situ hybridization demonstrated presence of GHRH transcripts in 5 of 9 human somatotrophinomas. In culture, GHRH-A failed to inhibit basal release of GH or production of cAMP irrespective of presence or absence of GHRH transcripts. However, GHRH-A was able to completely or partially abolish the stimulatory effects of exogenously added GHRH peptide. Additionally, the average stimulatory effect of exogenous GHRH on in vitro GH secretion by somatotrophinomas possessing GHRH mRNA was identical to that shown by tumors not expressing the GHRH gene. Whilst confirming that many human pituitary somatotrophinomas are able to express the GHRH gene, the failure of GHRH-A to inhibit basal GH secretion argues against the concept of the existence of an autocrine stimulatory loop involving secreted GHRH peptide.