In 581 patients with acute myocardial infarction (MI), the time lapse from estimated onset of infarction to estimated peak serum (S) enzyme activity was evaluated. Heat-stable lactate dehydrogenase (LD; E.C. 1.1.1.27) was analyzed every 12 h for 48-108 h after arrival in hospital (n = 581) and creatine kinase (CK; E.C. 2.7.3.2.; n = 224), and creatine kinase subunit B (CK-B; n = 211) were analyzed every 6 h for 48 h. Peak S-LD was observed between 14 and 110 h after estimated onset of MI (mean 46.6 +/- 0.6 h), peak S-CK was observed between 8 and 58 h (mean 25.0 +/- 0.6 h), and peak S-CK-B was also observed between 8 and 58 h (mean 22.8 +/- 0.7 h) after onset. In 86% of patients, peak LD was reached within 60 h after onset of MI, in 78%, peak CK was reached within 30 h, and in 82%, peak CK-B was observed within 30 h after onset of MI. A weak correlation was found between duration of pain and time lapse to S peak enzyme activity (r = 0.25 -0.27; p less than 0.001), while there was no correlation between S peak activity and time lapse from onset of MI to S peak activity. It is concluded, that although in the majority of patients with MI, peak serum-enzyme activity is reached within a predictable amount of time after estimated onset of MI there is wide variation, difficult to establish from the clinical course, among individual patients.