We have compared the haemodynamic effects of the imidazole derivative phosphodiesterase inhibitors enoximone and piroximone in patients with low cardiac output after cardiac surgery. Ten patients (group E) received enoximone and 10 patients (group P) received piroximone, both at a loading dose of 0.5 mg kg-1 followed by an infusion of 5 micrograms kg-1 min-1. In both groups the main changes with time were increases in cardiac index (maximum 24.5% in group E, 25.8% in group P) and decreases in systemic vascular resistance (maximum 26.8% in group E and 24.8% in group P). There were moderate increases in heart rate (maximum 11.3% in group E and 13% in group P) but a greater percentage decrease in mean arterial pressure in group E (maximum 11.9% vs 7.9%) with time. One patient in group E developed hypertension during the loading dose. Two patients in group E and two in group P developed hypotension during the loading dose. One patient in group E developed ventricular extrasystoles which may have been related to the drug being studied.