The ability of three mercury compounds to induce aberrations in the meiosis of mouse ova in vitro and in vivo was studied. Although threshold levels of each were shown in vitro to induce severe effects on the nuclei of ova, no immediate or delayed effect could be demonstrated in vivo. It is suggested that the mouse ovum is protected from the cellular effects of excess mercury by a barrier of unknown nature, perhaps residing in the zona pellucida, the vitelline membrane or even the follicular cells. No extrapolation to human ova is possible due to the known species variations in the metabolic fate of mercury.