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Summary. Impedance pneumography is based on the relationship between respired volume (ΔV) and changes of transthoracic impedance (Δ/Z/) during breathing. The impedance signal, the relationship between ΔV and Δ/Z/ and the success rate during long-term recording was studied in 11 healthy adult males, aged 21–46, by a portable tape recorder (Oxford Medilog 4–24). The signals from the tape recorder (Δ/Z/), and from a spirometer (ΔV) were recorded simultaneously on a mingograph during normal breathing, apnoea, deep sighs, and coughs. The relationship between ΔV and Δ/Z/ was also analysed on an X-Y recorder. Δ/Z/ was delayed in relation to ΔV and during one respiratory cycle the relationship was not linear. The time constant of Δ/Z/ was 6 s. A significant correlation was found between peak values of ΔV and Δ/Z/ for 0 < ΔKV<2·5 litres. Long-term recording of Δ/Z/ during one night's sleep resulted in a success rate of 63–100%, mean 90%. Invalidation of the impedance signal was caused by overloading (81%) and interference (19%). It is concluded that the Medilog respiratory recorder enables determination of respiratory frequency, periods of apnoea and a semiquantitative estimate of volume, but artifacts may invalidate the signal.