Purified nuclei isolated from mice challenged with nonlethal levels of mercury chloride (10-3M) in drinking water for 4 to 7 weeks (experimental) and from animals given deionized water (control) were fractionated and the subsequent fractions were analyzed for mercury by flameless atomic absorption. Control (active) euchromatin contained 1.75 ± 0.53 micrograms of mercury per milligram of DNA. There was a 12- to 15-fold enrichment of mercury in the euchromatin fraction of challenged animals. Mercury was not detected in control (inactive) heterochromatin, and only trace levels (parts per billion) appeared in experimental heterochromatin. It seems likely that mercury can be incorporated into chromatin as a metal-protein complex, but the possibility of protein-mercury-DNA or mercury-DNA complexes within euchromatin cannot be excluded.