Toxic effects of Hg^2+, Cd^2+ and Cr^6+ on developing chick embryos were examined by injecting the metal ions (1, 5, 10 and 50㎍ of each in saline solution) into yolk sac of fertilized egg. The injections were performed just before 3 days or 6 days after incubation. It was investigated on the enzyme activities (LDH, MDH, SDH) in liver, heart and pectoral muscle at 14 days after incubations. 1. The degree of decrease in the activities of LDH and MDH were greater than that of SDH. The relative sensitivity of the enzymes was MDH > LDH > SDH. 2. The toxicity of the metal inos is more heavy at early embryonic stage. The relative toxic effect of the metal inos was Cd^2+>Cr^6+>Hg^2+. 3. The relative sensitivity of tissues on the metal ions was liver > pectoral muscle > heart.