Interleukin-1 (IL-1) has been suggested to directly affect pituitary growth hormone (GH) release, although other investigators have failed to observe this effect. We examined the effects of IL-1 beta on GH secretion from single somatotrophs by means of reverse hemolytic plaque assay (RHPA). Anterior pituitary cells of adult male rats were enzymatically dispersed and subjected to RHPA. IL-1 beta at 100 pM and 1 nM, increased both the mean plaque area and the fraction of somatotrophs forming large plaques. IL-1 beta did not increase the mean plaque area in the presence of the IL-1 receptor antagonist (IL-1ra). IL-1 beta (1 nM) added together with GH-releasing hormone (GHRH; 10 nM), showed no additive effect on GHRH-induced GH release. The stimulatory action of IL-1 beta on the release of GH was suppressed by somatostatin. In conclusion, our data show that IL-1 beta stimulates GH-secretion through direct action on the pituitary.