Zinc-65 uptake and excretion by wild and laboratory small mammals differed in both the assimilation and excretion phases of the retention curves. The wild forms assimilated less of the ingested zinc and excreted the assimilated zinc at a faster rate than did the laboratory mouse and rat. The differences in rate of assimilation may be due to mode of administration; those in excretion may be a function of the differential activity or metabolism of the species under caged conditions.