The authors investigated the influence of glyceryl-trinitrate (NTG) given intravenously to the reduction of infarction size in 95 patients (71 men and 24 women) aged 36 to 75, with acute myocardial infarction (AIM) admitted to the Intensive Care Unit within six hours of the onset of pain. Infarction mass was calculated by mathematical model from the serial changes of CK and CK MB serum activities during 72 hours and expressed in CK and CK MB gEq. CK and CK MB were determined every four hours. The patients were divided into four groups according to the therapy they were receiving: I--NTG i.v. (n = 29); II SK + NTG i.v. (n = 29); III SK i.v. (n = 17) and IV ISDN per os (n = 20). Each group was divided into subgroups regarding the time interval from the onset of pain to the beginning of the therapy (within three hours and after three hours). Application of NTG i.v. in the early phase of AIM, 0-3 hours from the onset of pain, led to the significant reduction of infarction mass CK gEq and CK MB gEq (0-3 hours; middle rank = 11.35; 3-6 hours: middle rank = 17.7) (P < 0.05) and 0-3 hours: middle rank = 10.31; 3-6 hours: middle rank = 18.81 (P < 0.01). It was established that the "timing" factor was very important in the preservation of myocardial mass in AIM. It affirms the efficacy of NTG i.v., i.e., its direct effects on the coronary arteries and systemic effects that cause salvation of the myocardium. The influence of NTG iv to myocardial infarction size CK gEq did not depend on ECG localization. But it influenced the ECG localization when the infarction size was calculated from CK MB isoenzyme and expressed in CK MB gEq. Infarction mass CK MB gEq was statistically significantly smaller in the inferior than in the anterior localization (P < 0.05).