SUMMARY: The α-mannosidase activity and the zinc content of rat epididymis increased with age and there was a positive correlation between the two parameters. After castration of rats, epididymal α-mannosidase activity and zinc content both fell; subsequent administration of testosterone caused a partial restoration in enzyme activity, but not in zinc content. In zinc-deficient rats, the epididymal α-mannosidase activity reached little more than half the normal value for adults. In sucrose homogenates of rat epididymis and liver, α-mannosidase activity was concentrated in the cytoplasmic granules, but this was not accompanied by enrichment in zinc. In ovaries from cattle, pigs and women, the very low level of α-mannosidase activity in follicular fluid was reflected in a virtual absence of zinc: the zinc content of the corpus luteum was similar to that of the remaining ovarian tissue, in spite of the greater α-mannosidase activity of the former.