Ability to judge ventilation (V̇1) and respiratory rate (RR) was examined in healthy subjects using the standard psychophysical technique of magnitude production. The relationships between objective and subjective magnitudes of V̇1could be adequately described by power function with the mean exponent 1.9. The mean deviation from the regression line was 3.0%. The requested levels of V̇1were produced either by changing tidal volume (VT) or RR. Changes in VTresulted in higher exponents of V̇1judgment. When requesting the subjects to keep V̇1constant by matching either VTwith the imposed changes in RR or vice versa, the lesser acuity was found in adjusting RR to VTthan in matching VTwith RR. The results indicate that V̇1is judged on the basis of either VTor RR and is not treated as a concept in its own power. The ‘RR’ experiments consisted of assessment of RR and breath duration. The relationships between objective and subjective magnitudes of these two parameters were characterized by power functions with the exponents 1.0 and 1.2 respectively and mean deviation from the regression line of 13%. It is speculated that difficulties in adjusting RR to VTdepended on a lesser precision in RR