Proton spin relaxation induced by the triplet ground state of O-2 in the zinc-containing diamagnetic analogue of sperm whale deoxymyoglobin has been measured as a function of oxygen concentration. As no covalent binding of oxygen to the metal occurs in the zinc species, the relaxation effects of O-2 on the protein H-1 resonances arise exclusively via much weaker noncovalent interactions. The relaxation effects at the amide proton sites are found to be highly localized and are derived almost exclusively from O-2 binding at the four previously identified xenon binding sites. Relative binding constants of 1.0, 0.08, 0.07, and 0.23 were determined for the Xe 1, Xe 2, Xe 3, and Xe 4 sites, respectively. In combination with earlier measurements of the kinetics of the heme binding of oxygen, these equilibria measurements enable a more detailed analysis of models characterizing O-2 entry and egress. A correlation is established between the fraction of O-2 which enters the Fe2+-binding site via rotation of the distal histidine side chain (so-called "histidine gate") and the experimentally observable O-2 (or CO) lifetime in the Xe 1 site. A physiological role for these secondary oxygen binding sites is proposed in enhancing the efficiency of the O-2 association reaction by rendering more favorable its competition with water binding in the distal heme pocket.