In imaging of hyperpolarized noble gases, a knowledge of the diffusion coefficient (D) is important both as a contrast mechanism and in the design of pulse sequences. We have made diffusion coefficient maps of both hyperpolarized (3)He and (129)Xe in guinea pig lungs. Along the length of the trachea, (3)He D values were on average 2.4 cm(2)/sec, closely reproducing calculated values for free gas (2.05 cm(2)/sec). The (3)He D values measured perpendicular to the length of the trachea were approximately a factor of two less, indicating restriction to diffusion. Further evidence of restricted diffusion was seen in the distal pulmonary airspaces as the average (3)He D was 0.16 cm(2)/sec. An additional cause for the smaller (3)He D in the lung was due to the presence of air, which is composed of heavier and larger gases. The (129)Xe results show similar trends, with the trachea D averaging 0.068 cm(2)/sec and the lung D averaging 0.021 cm(2)/sec. Magn Reson Med 42:721-728, 1999.