Abstract: A method for determining the tidal volume (Vt) distribution in the lungs of a subject. The distribution is made among a plurality of compartments (i). Each compartment has an assigned tidal volume (Vti)/perfusion (Qi) ratio and an assigned tidal volume (Vti)/compartmental volume (Vai) ratio so that the compartments, collectively, cover a range of ventilation/perfusion ratio (Vt/Q) values and ventilation/volume (Vt/Va) ratio values. The subject inspires a bolus of analytical gases in his/her tidal volume of breathing gases. The analytical gases of the bolus have differing solubilities in the blood of the subject and are inspired in known amounts. The concentrations of the analytical gases expired by the subject are measured, preferably in a plurality of breaths. The expired analytical gas concentrations are expressed as a function dependent on the inspired bolus amounts, the ventilation/perfusion ratios, the ventilation/volume ratios, and the compartmental tidal volumes (Vti).