We measured the velocity and attenuation of audible sound in the isolated lung of the near-term fetal sheep to test the hypothesis that the acoustic properties of the lung provide a measure of the volume of gas it contains. We introduced pseudo-random noise (bandwidth 70 Hz - 7 kHz) to one side of the lung and recorded the noise transmitted to the surface immediately opposite, starting with the lung containing only fetal lung liquid and making measurements after stepwise inflation with air until a leak developed. The velocity of sound in the lung fell rapidly from 187 ± 28.2 to 87 ± 3.7 m.s^-1 as lung density fell from 0.93 ± 0.01 to 0.75 ± 0.01 g.ml^-1 (lung density = lung weight/gas volume plus lung tissue volume); for technical reasons no estimate of velocity could be made before the first air injection. Thereafter, as lung density fell to 0.35 ± 0.01 g.ml^-1 there was a further decline in velocity to 69.6 ± 4.6 m.s^-1. High frequency sound was attenuated as lung density decreased from 1.0 to 0.5 g.ml^-1, with little change thereafter down to a density of 0.35 ± 0.01 g.ml^-1. We conclude that both the velocity of audible sound through the lung, and the degree to which high frequency sound is attenuated in the lung, provide information on the degree of inflation of the isolated fetal lung, particularly at high lung densities. If studies of sound transmission through the lung in the intact organism were to confirm these findings, the acoustic properties of the lung could provide a means for monitoring lung aeration during mechanical ventilation of newborn infants.