This study was designed to study the role of adenosine in ischaemic preconditioning. The clinical, electrocardiographic and metabolic parameters of ischaemia were compared before and after intracoronary adenosine during percutaneous transluminal coronary angioplasty in 15 patients with significant stenosis of the left anterior descending artery. Baseline ischaemia was demonstrated by inflating the balloon of appropriate size at its nominal pressure for 30 seconds. Intracoronary adenosine was then administered in dose of 1000 to 6000 mgm/ml. The balloon was repositioned and inflated at nominal pressure for 180 seconds. As compared to 1st balloon inflation, the 2nd balloon inflation produced less severe chest pain, less ST segment deviation on the intracoronary ECG (7.40 +/- 3.94 vs 4.30 +/- 2.15 mm; p < 0.05) and lower coronary sinus lactate levels as compared to the first (0.28 +/- 0.06 vs 0.22 +/- 0.04 mmol/lit; p < 0.05). In conclusion, this study proves that by injecting intracoronary adenosine it is possible to precondition the human myocardium.