The effects of mercuric compounds on the proliferation and protein synthesis of mouse blastocysts were examined in vitro by treating the embryos with methylmercury (MMC) or mercuric chloride (MC). Late blastocysts were exposed to various concentrations of mercuric compounds for 24 hr. and incubated for another 24 hr. in a mercury-free medium. The protein synthesis of the mercury-treated blastocysts was measured by counting the incorporation of L-[35S]methionine into the acid-insoluble protein fraction of a cell. 0.1 microM of MMC was equivalent to 20 microM of MC with regard to the inhibitory effect on proliferation, and equivalent to 2 to 5 microM of MC after 24 hours' culture, and 10 to 20 microM after 48 hours' culture with regard to the inhibitory effect on protein synthesis of the blastocysts. The results showed that MMC was approximately 200 times more toxic than MC with regard to cell number proliferation, and 20 to 50 times so with regard to protein synthesis capability.