The errors in electrical impedance plethysmography are not well known. In the paper errors due to inadequate instrumentation, improper electrode application, and physiological changes have been analysed. Minimum electrical requirements for a general purpose four-electrode instrument are: (a) input impedance of voltage-sensing circuit+4 kΩ; (b) output impedance of constant-current source+20 kΩ; (c) c.m.r.r.+400; (d) common-mode input impedance+40 kΩ; and (e) apparent change of resistance due to noise×0·5 mΩ. All electrodes should be separated by at least one limb diameter. Changes of electrode impedance due to pressure produce negligible error in a four-electrode configuration. The only way to eliminate the large subject-motion errors is to eliminate the motion. Changes of blood resistivity with velocity produce an error estimated at 10 per cent. A repeatability test on the upper arm gave a standard deviation of 6 per cent of the mean.