The ability to depress the resonance intensity of (23)Na in rat liver tissue was not found in the supernatant fraction. It was exclusively localized in particulate fractions. The intensity and saturation behavior of the (23)Na signal was examined in suspensions containing various amounts of the particulate fraction of rat liver homogenate. The results strongly suggest that the (23)Na signal of tissue reflects quadrupole interactions and does not result from a slow exchange between the free and bound fractions of Na(+). The activity coefficient of Na(+) in rat liver homogenate (no medium was added) was 0.59, about 20% less than that in the isotonic saline. Available evidences and discussion indicate that the bound Na(+) in the homogenate is much less than the so-called "NMR-invisible" fraction of Na(+).