1. We studied the acute haemodynamic effects of cromakalim, a vasodilator which activates smooth muscle potassium channels, in 11 patients with ischaemic heart disease undergoing routine cardiac catheterisation. A similar group of six patients given placebo were studied under identical conditions. 2. There were no significant differences in baseline haemodynamic parameters between the two groups. 3. Following intravenous cromakalim (15 micrograms kg-1) cardiac output increased by 30% (P less than 0.05 vs placebo) while systolic arterial pressure decreased by 8% (P less than 0.05), systemic vascular resistance decreased by 29% (P less than 0.01) and pulmonary vascular resistance decreased by 24% (P less than 0.01) at plasma concentrations of the (+)- and (-)-enantiomers of cromakalim of 6.2 +/- 0.5 ng ml-1 and 10.0 +/- 1.0 ng ml-1 respectively. 4. There were no significant differences in diastolic arterial pressure, left ventricular dP/dt and stroke volume between the two groups. Heart rate increased by 11% following cromakalim but this did not achieve significance. 5. These findings confirm that cromakalim acts primarily as an arteriolar vasodilator producing an improvement in cardiac performance. Cromakalim may be of benefit in the treatment of patients with ischaemic heart disease.