The reproducibility of myocardial ischaemia provoked by atrial pacing was studied in two groups of patients. The heart rate was slowly increased (10 beats per fourth min) until angina in group A, and quickly increased (10 beats per 20 s) in group B. Cardiac venous flow was measured by thermodilution and blood was sampled for metabolic studies at rest and during the maximum obtained heart rate and repeated after 20 min of rest. Ischaemia was defined as a reduction in myocardial lactate extraction ratio by 50% and to a ratio lower than 0.15. Lactate metabolism changed from production (-0.06 ± 0.05) during the first pacing to extraction (0.05 ± 0.05) during the second in group A (p < 0.02). Lactate metabolism was quite reproducible in group B. All the other metabolic and haemodynamic parameters were unchanged in both groups. We conclude that the duration of atrial pacing is important for the reproducibility of pacing-induced myocardial ischaemia.