Changes in growth hormone-releasing hormone (GHRH), GHRH-receptor (R), somatostatin and interleukin (IL)-1beta mRNA levels were determined in fetal rat hypothalamic cultures after administration of IL-1beta (1, 10, 100 ng/ml, 2 h incubation), and in adult rat hypothalamus 5 h after intracerebroventricular injection of IL-1beta (2.5 and 25 ng). IL-1beta stimulated GHRH-R mRNA expression both in vitro (10 and 100 ng/ml) and in vivo (2.5 and 25 ng). Somatostatin mRNA was significantly stimulated and GHRH mRNA slightly reduced in vitro, while these mRNA species were not altered in vivo in response to IL-1beta. IL-1beta stimulated its own expression both in vitro (10 and 100 ng/ml) and in vivo (25 ng). IL-1beta-induced mRNA responses occurred 2 h after treatment in vitro (incubation times, 30 min to 6 h). IL-1beta also elicited slight GHRH releases in vitro. Up-regulation of hypothalamic GHRH-R by IL-1beta may explain previous findings suggesting that IL-1beta stimulates GHRH activity.