The effects of altering the integrity of the chorion following various treatments were examined in rainbow trout (Salmo gairdneri) embryos. The acellular chorion which normally hardens into a tough resilient capsule on contact with water can be rendered up to 200 fold softer by brief treatment immediately after fertilization with the mercury containing compounds, sodium mersalyl or mersalyl acid. Alkaline conditions greatly enhance the softening action of these compounds. Tris buffer at pH 10 resulted in some degree of softening while at pH 6 hardening greater than normal was achieved. Development in the softened eggs appeared unaffected except that hatching was accelerated. The extent of premature hatching was found to depend on the degree of chorion softening. While softened eggs were more sensitive to adverse conditions, almost normal embryonic survival could be attained with careful rearing. Embryos of softened eggs did not exhibit increased sensitivity to various exogenous agents suggesting that permeability remained unaltered.