In 34 patients with coronary atherosclerosis a pacing test was performed with measurement of the lactate, glucose, potassium and inorganic phosphate coronary arterio-venous differences. Eighteen of these 34 patients felt no pain during the pacing test. In this group of asymptomatic patients, there was no significant change of the lactate, glucose, potassium and inorganic phosphate myocardial extraction. In the 16 patients who felt an anginal pain during the pacing test, there was a significant myocardial production of lactate, but the myocardial loss of potassium and inorganic phosphate was not continuously statistically significant. Myocardial extraction of glucose during the pacing-induced angina did not increase. Although, in some patients, both the potassium and the inorganic phosphate might be used to assess a condition of myocardial ischaemia, the lactate remains the best metabolic criterion for pacing-induced ischaemia.