Dobutamine, a recently introduced derivative of dopamine, is reported to retain inotropic properties with less pronounced chronotropic and arrhythmogenic effects than isoproterenol. The drug was evaluated in two doses, 5 μg/kg/min and 10 μg/kg/ min, in two groups of ten patients each, during emergence from cardiopulmonary bypass. A third group of five patients was studied similarly with isoproterenol, 0.02 μg/kg/min. Cardiac index increased 16 and 28 per cent with the two doses of dobutamine, respectively, and 9 per cent with isoproterenol. Heart rate, in contrast, increased 6 and 15 per cent with dobutamine (not significant) and 44 per cent with isoproterenol (significant). Dobutamine seemed to be associated with fewer arrhythmias than isoproterenol. It is concluded that dobutamine, 5–10 μg/kg/min, is suitable for use during emergence from cardiopulmonary bypass and may possess advantages over isoproterenol.