The effects of recombinant tissue plasminogen activator (rt-PA) and urokinase on patency and early reocclusion of infarct-related coronary arteries were investigated in a single blind, randomised multicenter trial in up to now 125 patients with acute myocardial infarction of less than six hours duration. Both, 70 mg of single-chain rt-PA with an initial bolus of 10 mg, and 3 million U of urokinase with an initial bolus of 1,5 million U were given intravenously over 90 minutes. The first angiogram at the end of the infusion revealed a patent infarct-related artery (TIMI grade 2 or 3) in 68% of 62 patients with rt-PA vs. 63% of 63 patients with urokinase (n.s.). Twenty-four hours later patent infarct-related arteries occurred in the same frequency in the rt-PA group and in the urokinase group (71.5% vs. 74.6%, n.s.), although additional recanalisation procedures in sequence with the first angiography were performed more frequent in the rt-PA group. There were two cardiac deaths in either group. In-hospital reinfarction rate was 9.7% vs. 17.5% for patients treated with rt-PA and urokinase, respectively. Both drugs were well tolerated, no significant difference of cardiovascular or bleeding complications could be observed between the two groups. It is concluded that rt-PA and urokinase in the dosages used provide similar efficiency and safety in the treatment of acute myocardial infarction.