[en] Immediately after spawning had occurred in the early stage of exposure experiment, the female individuals exposed to ⁵⁴Mn concentrated more actively the nuclide than the males, and retained it in whole bodies at 1.70 times higher compared to the males in 100 %-SW. Moreover, the biological half-life estimated from the excretion formula was 1.58 times longer in the females than in the males, i.e. ⁵⁴Mn in the females was eliminated more gradually than that in the males. These results differed clearly from those obtained on H. discus at the pre-spawning stage and furthermore, from stable-Mn distribution observed in females and males of Mytilus edulis at the post-spawning season presented previously elsewhere. ⁵⁴Mn distributed to various soft tissues in the females were apparently higher than those in the males at the end of the exposure in 100 %-SW. The concentration factor for ovary of specimens in 100 %-SW raised up to 990 at the end of the exposure, while that of spermary was only 146. The difference between that for the ovary and the spermary was of the highest value among tissues, and consequently a similar correlationship existed in absolute amounts and concentrations. The distribution of ⁵⁴Mn to ovary exhibited similar trend at the pre- and the post-spawning stages, but its degree differed from each other and was enhanced in concentration at the latter stage. The nuclide uptake, retention and excretion by the females were inhibited under depressed specific gravity condition, whereas the males were not affected at all. (author)