Seven patients with right precordial Q waves (RPQs) simulating anterior myocardial infarction (MI), but without a previous history of either MI or other underlying diseases showing RPQ, were injected intravenously with ajmaline. The RPQs were abolished in 5 patients. Two patients displayed a transient, spontaneous abolition of RPQ, and 2 patients had false positive ST responses to the exercise test. Ajmaline injection in patients with documented MI and in patients with normal electrocardiograms did not produce any notable changes in the initial part of QRS complex. The RPQs abolished by ajmaline were probably a result of a mild preexcitation, induced by Mahaim fiber conduction, because the PR and QRS intervals were normal and low amplitude delta waves were found in some patients. It is important to note that altered intraventricular conduction, probably a mild preexcitation, can markedly deform electrocardiograms and mimic myocardial infarction.