AbstractRana pipiensembryos at the cleavage, blastula, gastrula, and neural‐plate stages of development were treated with methylmercuric chloride in concentrations of 0.5–200 parts per billion (ppb) to determine embryocidal and teratogenic effects. Concentrations of 40 ppb and above were lethal to embryos treated during the cleavage stage. Embryos at the blastula, gastrula, and neural‐plate stages were treated for 5 days with 5–30 ppb; 5 ppb had only minor effects, 10, 15, and 20 ppb caused various defects (exogastrulae, poor tail development, poor general development) and increased death rates related to exposure time and concentration, and embryos treated with 30 ppb had a high frequency of defects after 24 h and all died by the end of the 3rd day. Individual tolerances to methylmercury of various degrees were o