The effects of aprindine, 100 mg iv, on hemodynamics, and the relationship between its inhibitory effect on PVC and its levels in the blood were determined in patients with diminished cardiac function. PVC was inhibited in 7 of 13 patients (54%), compared with a 50% inhibition rate in controls. The levels of aprindine in the blood after intravenous administration, rapidly decreased from 1.78 +/- 1.09 micrograms/ml immediately after administration, to 0.80 +/- 0.25 micrograms/ml after 15 min, to 0.65 +/- 0.23 micrograms/ml after 30 min and to 0.56 +/- 0.19 micrograms/ml after 1 hour. The duration of blood levels of 0.55 +/- 0.35 micrograms/ml, which are the levels presumed to be effective, was one hour after administration. The mean elimination half-life of aprindine was 18.9 +/- 8.4 hours. Aprindine produced relatively little effect on hemodynamics in patients with moderate to severe heart failure, but when its effects in individual cases were studied, it was found that aprindine elicited such changes as reduction in cardiac index, stroke volume index and stroke work index, and elevation in pulmonary arterial diastolic pressure. These findings suggest that care should be exercised in aprindine therapy in patients with diminished cardiac function. At least there should be monitoring of blood pressure and heart rate at appropriate times after intravenous administration.