To determine whether sustained plasma concentration of human tissue-type plasminogen activator (t-PA) can be induced promptly after intramuscular injection with enhancers of absorption devoid of deleterious local and systemic effects, we studied 250 rabbits and 13 dogs. In rabbits with t-PA injected directly into exposed muscle followed by local electrical stimulation at the site, early absorption was increased markedly by addition of 0.63M methylamine plus 0.079M hydroxylamine to the excipient. Elevations peaked within 5 min and increased with dose of t-PA, concentration of methylamine, and volume of injection medium. The enhancers were effective with percutaneous injections in the absence of local electrical stimulation as well. They did not elicit any obviously deleterious local or systemic effects. In separate experiments in rats, intramuscular injections of 0.63M methylamine plus 0.079M hydroxylamine induced local egress of intravascular radiolabeled albumin within the injection site and endothelial gaps in venules detected with colloidal carbon--changes consistent with direct effects on vascular permeability. In dogs, percutaneous intramuscular injection of t-PA in excipient without enhancers did not lead to early elevations of human t-PA in plasma, although late elevations were seen. When the enhancers were used, early elevations occurred as well, with functional activity documented by fibrin plate assays of serially obtained plasma samples and by sequential coronary angiography delineating thrombolysis after experimentally induced coronary thrombosis. The results indicate that intramuscular administration of t-PA with selected enhancers of absorption is a feasible approach for rapid induction of fibrinolysis.