The gross, histological, and selected chemical effects on developing chick embryos resulting from injection of thallous salts into the yolk sac have been described. A specific time-related and dose-related alteration of the embryonal cartilage of the long bones follows this treatment and results in a characteristic skeletal deformity. In the chick embryo, the skeletal cartilage appears to have a tissue-specific susceptibility to certain dosages of thallous salts. Chondrocyte necrosis, defective chondrocyte maturation, and, perhaps, an alteration in the metabolic pathways of mucopolysaccharide synthesis appear to be caused by a direct cytotoxic effect of thallous salts under the conditions of these experiments.