13N2 and 81Krm were used to measure changes in upper/lower (U/L) ratios of regional ventilation in supine anaesthetised dogs mechanically ventilated at different tidal volumes and the results compared. Analysis of the slopes of the 13N2 washout curves indicated that U/L specific ventilation was less than unity at low tidal volumes and decreased as tidal volume increased. U/L total ventilation, measured using the specific ventilation results and estimations of relative lung using 13N2, also decreased with increase in tidal volume. Preferential distribution of ventilation to dependent lung regions thus increased with tidal volume despite the opposing hydrostatic forces from the abdomen. Regional count density distributions in 81Krm images of the lungs were analysed. At high specific ventilation rates, correction of 81Krm count density is required in order to obtain correct regional ventilation information. The specific ventilation rates in this series were such that the uncorrected U/L count ratios of 81Krm increased as tidal volume increased in contrast to the decrease in U/L total ventilation recorded by the 13N2 method. When corrections using the 13N2 specific ventilation results were applied, the changes in U/L 81Krm count ratios with tidal volume showed a similar trend to the changes in total ventilation measured with 13N2. Sources of error in the two techniques are briefly discussed.