The effect of osmotic changes on fertilized mouse ova was studied by measuring their survival, defined as development into hatching blastocysts, after exposure to various concentrations of ethanediol (ethylene glycol). In addition, a Boyle-van't Hoff plot was derived from exposing ova to hypotonic and hypertonic solutions ranging from 0.1 to 2.8 osmol. Volume of ova was inversely proportional to osmolality over this range. Extrapolation of this relationship yielded a nonosmotic volume of the ova of 22.5%. Eighty-five per cent or more of the ova survived exposure to this wide range of concentrations and developed into blastocysts. The rate of development of ova exposed to anisotonic solutions was the same as that of controls. Ova underwent osmotic shock when abruptly diluted out of concentrated solutions of ethanediol with an isotonic solution. Their survival was highly dependent on the ethanediol concentration with which they had equilibrated before dilution, and the manner, rate and temperature of dilution. The longer the exposure to ethanediol the greater was the sensitivity of the ova to osmotic shock, reflecting permeation of ethanediol into the ova. Osmotic shock could be alleviated by dilution at a high temperature, and prevented by the use of sucrose as an osmotic buffer at 37 degrees C. Identification of the variables that influence osmotic shock of ova will be helpful in the systematic study of their cryopreservation.