Human ability to detect small changes in the inspired volume was tested in four healthy subjects. The tested volume changes were achieved by: 1) displaying breathing volume continuously to the subject on the screen of an oscilloscope, the vertical gain of which was changed by the experimenter, and 2) breathing small amounts of CO2(0.02–2.0%) in air. The subject was asked to signal, by pressing a button, every time two consecutive breaths differed from the reference level. The six tested volume changes corresponded to 15–65% of the reference, when changes in eupneic tidal volume were tested, and to 15–45% when changes in three reference volume levels, 1.0, 1.5, and 2.0 times the resting tidal volume, were tested. The just noticeable difference (JND) was defined as the volume change detected in 50% of the trials. The obtained results showed that the JND for volume corresponds to a volume change of 120–160 ml (23–28% of the reference) when the resting tidal volume was used as the reference. For the three reference volumes, the JND was found to be a nearly constant fraction (0.25–0.29) of the reference volume and thus followed the ‘classical’ Weber law (JND/reference stimu