The aim of this study was to characterize the effect of alveolar macrophage (AM) secretory products on endothelin (ET)-1 production by rat alveolar type II (ATII) cells in primary culture. We quantified preproendothelin (ppET)-1 mRNA by Northern blot and ET-1 concentrations in cell supernatants by enzyme-linked immunosorbent assay. Conditioned medium (CM) from rat adherent AM decreased the ppET-1 mRNA levels in ATII cells and reduced ET-1 concentrations in cell culture supernatants. This effect was mediated by interleukin (IL)-1 beta as shown by pretreatment of CM with an anti-IL-1 beta neutralizing antiserum. IL-1 beta effect was dependent on protein synthesis, was partially prevented with indomethacin, and was totally prevented with dexamethasone. Specific inhibition of cyclooxygenase 2 activity completely reversed the effect of IL-1 beta. We conclude that rat AM inhibit ET-1 production by rat ATII cells in vitro through IL-1 beta secretion. The IL-1 beta-mediated inhibition is dependent on the cyclooxygenase 2 pathway. Downregulation of ET-1 production by activated AM could limit the intra-alveolar burden of this profibrogenic peptide and thus could prevent fibrosis development.