OBJECTIVE The aim was to assess the effects of therapeutic doses of intravenous adenosine on human atrial and ventricular repolarisation. METHODS The effects of 6 mg and 12 mg bolus doses of adenosine on the atrial and ventricular monophasic action potentials were studied using the contact catheter technique in 19 patients undergoing routine diagnostic electrophysiology studies. The effect on atrial repolarisation was studied before and after beta blockade in a subgroup of patients. RESULTS The duration of the monophasic action potential to 90% repolarisation (MAPD90) was measured in all cases. After 6 mg of adenosine the atrial MAPD90 shortened from 227(SD 29) ms to 188(25) ms (p < 0.005); after 12 mg it shortened from 221(31) ms to 168(32) ms (p < 0.001). The maximum shortening was unaltered by propranolol 0.15 mg.kg-1. The ventricular MAPD90 showed no significant change after 12 mg, at 240(32) ms v 234(33) ms. CONCLUSIONS Therapeutic doses of adenosine shorten the atrial but not the ventricular monophasic action potential duration. The effect is dose dependent and not abolished by beta blockade.