The aim of this study was to explore the chronergy of intravenous recombinant streptokinase (r-SK) in patients with acute myocardial infarction (AMI). A total of 114 patients were divided into two groups according to the time of AMI onset: the morning onset (6:01-12:00, n=53) and non-morning onset (12:01-06:00, n=61) groups. The recanalization rate was recorded, as well as anticoagulant and fibrinolytic indices. Statistical analysis was performed to evaluate the recanalization rate following thrombolysis, as well as the anticoagulant and fibrinolytic activities. The recanalization rates following thrombolysis in the morning onset and non-morning onset groups were 60.4 and 82.0%, respectively (P<0.05). The level of plasminogen activator inhibitor-1 (PAI-1) antigen was significantly higher in the morning onset group compared with. that in the non-morning onset group (P<0.05). This indicated a resistance to r-SK thrombolysis in the morning at the early stage of AMI, which possibly correlates with increased PAI-1 antigen levels and activity.