We have studied two new temporary pacing leads (Medtronic 6491 and 6492) intended for pacing after cardiac operations. The conductor has stainless steel strands coated with polyethylene connected to a 4' mm2 surface area, stainless steel, smooth, tapered electrode. A soft 4-0 coiled polypropylene fiber served as as fixation mechanism in the heart. The study included 15 children (aged 3 months to 7 years, body weight 4.4 to 20 kg) with a variety of congenital heart defects and 15 adults (aged 45 to 78 years) with coronary artery disease (n = 13) and aortic valve disease (n = 2). A pair of leads each was placed in a bipolar fashion in the right atrial wall and nonsystemic ventricle in the children (median implant duration 12 days) and in the right atrial wall only in the adults (median implant duration 9 days). The atrial current threshold values in children increased from 0.61 +/- 0.34 mA immediately after implant to 2.08 +/- 1.86 mA at explant (p < 0.002). In the adults the threshold values increased from 0.95 +/- 1.44 mA immediately after implant to 2.76 +/- 2.76 mA at explant (p < 0.002). In the ventricle the threshold values increased from 0.38 +/- 0.13 mA immediately after implant to 2.22 +/- 1.63 mA at explant (p < 0.002). Tissue resistance immediately after implant measured 809 +/- 182 omega at explant (children, p = not significant). Corresponding values in adults were 778 +/- 190 omega and 599 +/- 91 omega (p < 0.004). In the ventricle resistances changed from 1019 +/- 143 omega to 876 +/- 137 omega (p < 0.05). P wave amplitudes measured 1.8 +/- 1.5 mV immediately after implant and decreased to 1.6 +/- 1.2 mV at explant (p = not significant, children) and 2.0 +/- 1.3 mV to 1.8 mV (p = not significant, adults). R wave amplitude were 13.1 +/- 3.0 mV immediately after implant and fell to 8.7 +/- 4.5 mV at explant (p < 0.005). Thus, threshold values, tissue resistances, and electrogram and pliable amplitudes assured a safe pacemaker function. The small diameter and pliable texture of these leads provided a smooth surgical handling. They were found particularly suitable in children.