Praecordial ECG-mapping (forty-two unipolar leads) was carried out on sixteen patients with anterior acute myocardial infarction (AMI) and on ten control patients without AMI. The variability, chronometric changes and the error-of-measurement when measuring sigma ST, sigma Q and sigma R in an ECG-map were evaluated. The investigation demonstrated that there was a significant inter-individual variation of sigma ST, sigma Q and sigma R in both patient groups. In cases of AMI, the intra-individual variation of sigma St, sigma Q and sigma R was significantly greater than the variation in the error-of-measurement; this applied also to the control group, except for sigma ST which was of the same magnitude. Inducing measuring errors, which were brought about by changes in the position of the patient, were of the same magnitude as the itra-individual variation in the control group. The chronometric variation of sigma ST, sigma Q and sigma R were significant in the patients with AMI; this also applied to sigma ST in the control group, but not to sigma Q and sigma R. In patients with AMI, sigma ST and NST were correlated to heart rate, mean arterial blood pressure, the product of heart rate and systolic blood pressure, and respiratory rate. However, only the correlations between heart rate and sigma ST and NST, respectively, were of such a magnitude that they were of clinical relevance. Thus 37-42% of the variance in the ST segment within 14 days after the onset of the infarction can be explained by changes in the heart rate.