The efficacy of thyroxine against methylparathion poisoning in chick embryos was studied. The mortality rate and survival rate, frequency of abnormalities, growth rate and size of embryos, and also the change in cholinesterase activity were determined to evaluate the protective effect of thyroxine and atropine. It was observed that the survival rate, growth rate and size, and the cholinesterase activity significantly declined in the methylparathion treated group while the mortality rate and the frequency of abnormalities increased. When thyroxine was given, a significant reversal in these parameters was seen, indicating an effective protective action of thyroxine against methylparathion intoxication in chick embryos. The results also showed that the therapeutic treatment of the combination of thyroxine and atropine did not further improve the effects. Since in many respects, chick embryo development parallels that of mammalian embryos, a short term use of thyroxine as an effective protective agent against organophosphate methylparathion (perhaps other compounds) poisoning may have important implications.