The monokine interleukin-1 (IL-1) inhibits endothelial cell growth and induces prostacyclin production in human endothelial cells. Since cyclooxygenase (Cox) is the rate-limiting enzyme in the synthesis of prostanoids, we evaluated the ability of IL-1 to stimulate Cox expression by human umbilical vein endothelial cells (HUVEC) in vitro. Our data demonstrate that 1) the Cox mRNA is expressed at low levels in untreated cells; 2) IL-1 alpha induces the Cox mRNA within 2 h, and this induction is sustained for more than 24 h; 3) IL-1 alpha induction is dose-dependent; 4) cycloheximide potentiates the induction of the Cox mRNA by IL-1 alpha while actinomycin D prevents the induction, and 5) IL-1 alpha also stimulates Cox production in a time-dependent fashion which correlates with the increase in prostacyclin synthesis. These data suggest that Cox is an immediate-early gene induced by IL-1 in HUVEC and may contribute to the regulation of the endothelial cell differentiation pathway in vitro.