Conditions for minimum energy pacing of the heart have been investigated. A total of 99 patients undergoing permanent cardiac pacing with one of three different leads were studied. Measurements of threshold voltage and current flow were made at 0.25, 0.5, 0.75, and 1.0 ms pulse durations. All measurements were made directly from the lead at implantation. There were no significant differences in threshold energy between the three leads used, and therefore all 99 leads were pooled for further analysis. There were significant differences (P less than 0.05) between the pacing energies required at the four pulse durations; the mean threshold energy at 0.25 ms was 156 nJ, at 0.5 ms 115 nJ, at 0.75 ms 110 nJ, and at 1.0 ms 102 nJ. Significant energy savings may be achieved when programming pacemakers to the optimum output for each patient by reducing pacemaker voltage - current output in preference to reducing pacing pulse duration.