Type 1 plasminogen activator inhibitor (PAI-1), a major physiological inhibitor of plasminogen activation, is an important component of the hepatic acute phase response. We studied the acute phase regulation of murine hepatic PAI-1 in response to systemic toxicity and local tissue injury in both wild-type mice and in mice in which the interleukin (IL)-1 gene had been inactivated by gene targeting. Endotoxin induced plasma PAI-1 antigen levels and PAI-1 mRNA accumulation in liver to the same extent in both wild-type and IL-1-deficient mice. In contrast, turpentine increased plasma PAI-1 and hepatic PAI-1 mRNA accumulation in wild-type mice but not in IL-1-deficient mice. Intraperitoneal injection of murine IL-1 rapidly increased plasma PAI-1 and hepatic PAI-1 mRNA in both wild-type and IL-1-deficient mice. These results suggest that IL-1 is a critical inducer of hepatic PAI-1 gene expression during the acute phase response to local tissue injury. In situ hybridization studies revealed that hepatocytes are the cells primarily responsible for the hepatic expression of the PAI-1 gene induced by lipopolysaccharide and turpentine.