Nuclear magnetic resonance studies of 129Xe are consistent with one kinetically distinguishable binding environment in methemoglobin and two in metmyoglobin. The Xe binding site in methemoglobin is assigned to a cavity formed by the A-B and G-H corners of the globin chain [Schoenborn, B.P. (1965) Nature (London) 208, 760-762]. The small differences between alpha-hemoglobin and beta-hemoglobin are not resolved by the NMR experiments. The Xe association rate constant at 18 degrees C with methemoglobin is greater than 6 X 10(7) M-1 s-1 with an activation barrier of approximately 13 kcal/mol. One of the binding sites in metmyoglobin associated with a cavity on the proximal side of the porphyrin ring, opposite the O2 binding site [Schoenborn, B.P., Watson, H.C., & Kendrew, J.C. (1965) Nature (London) 207, 28-30]. An estimate of the association rate constant of Xe at 18 degrees C is 1 X 10(7) M-1 s-1 with an activation barrier of approximately 16 kcal/mol. The second metmyoglobin binding site has similar NMR and kinetic properties of those for methemoglobin.