Diurnal variation and reproducibility of abnormalities occurring during predischarge postinfarction treadmill exercise testing were investigated in 41 patients. Each patient was exercised using a limited Naughton protocol before 0800 and after 1800 h on two consecutive days. No complications arose. Individual ischaemic abnormalities were poorly reproducible in any patient. No abnormality and no patient showed significant diurnal variation. When the presence of any one of three ischaemic abnormalities (ST segment depression or elevation and angina) was analysed the reproducibility of an ischaemic result in the two morning and the two evening tests was 72% and 95% respectively with no significant difference between the two. The reproducibility of an ischaemic result in all four tests was 66%. The reproducibility of the test for either the presence or absence of an ischaemic result was 71%. No training effect could be shown either for the group as a whole or for any individual patient. There were no appreciable differences in either the heart rates or systolic blood pressures during exercise among those patients with non-reproducible ischaemic test results. Thus it is concluded that an assessment of any one of three ischaemic abnormalities improves the reproducibility of the result of submaximal exercise testing after infarction. Mechanisms other than increased myocardial oxygen consumption related to increased heart rate and systolic blood pressure at submaximal exercise tolerance are needed to explain non-reproducible ischaemic test results.