The effects of thyroxine and atropine in ameliorating phosphamidon intoxication in chick embryos was studied. Treatment of phosphamidon significantly enhanced the mortality and abnormality rates, decreased the average body weights, and cholinesterase activity in chick embryos. When thyroxine was administered to the phosphamidon intoxicated embryos, the above parameters changed significantly, indicating an ameliorating effect of thyroxine against phosphamidon intoxication in chick embryos. The combined thyroxine and atropine therapy did not further improve the ameliorating effect. Since in many respects chick embryo development parallels that of mammalian embryos, a short-term use of thyroxine as a protective agent against organophosphate toxicity might be useful.