1. The aim of the present study was to examine, in the dog myocardium, the incidence of zig-zag mitochondrial cristae over time in the non-ischaemic posterior wall, following an acute anterior wall infarct. 2. Changes within the myocardial mitochondrial crista membrane in dogs were investigated following acute left anterior descending coronary artery ligation. Transmyocardial biopsy samples were taken serially from the posterior non-ischaemic wall in the same dog. Changes in heart mitochondrial cristae were examined by transmission electron microscopy prior to coronary ligation (control) and 40 min and 2, 4, 6 and 24 h postinfarction. 3. In control hearts, 90% of mitochondrial cristae had a lamelliform appearance. Following infarction, there were two transitional states with respect to mitochondrial cristae, the first characterized by undulating lamelliform cristae that are also found in 10% of control samples and a second transitional state that was zig-zag and reached a maximum between 6 and 24 h after infarction. 4. In conclusion, an undulating lamelliform crista pattern is present in the non-ischaemic wall of the acute infarcted dog and we hypothesize that this may be an intermediate from, between 'normal' lamelliform and 'abnormal' zig-zag cristae.