The RT-PCR technique was used to study IL-1 receptor mRNA levels in AtT-20 cells. IL-1 beta increased type I IL-1 receptor mRNA levels within 1 h, with elevated levels remaining after 24 h, whereas it induced a bell-shaped alteration of type II IL-1 receptor mRNA levels, with a peak after 6 h. Tumour necrosis factor-alpha (TNF alpha) similarly up-regulated type II IL-1 receptor mRNA levels, and type I IL-1 receptor mRNAs albeit to a lesser extent than IL-1 beta. Furthermore, IL-1 beta also induced increases in TNF alpha and c-fos mRNAs. The IL-1 receptor antagonist can fully block all the above effects of IL-1 beta. Up-regulation of type II IL-1R mRNA levels in AtT-20 cells could constitute an important way to modulate IL-1 actions, since type II IL-1R is believed to antagonize IL-1 effects.