The coefficients of permeability to glycerol, Pglyc, and their temperature-dependence were determined from volume changes of fertilized and unfertilized mouse ova under hyperosmotic conditions. Fertilization caused a significant increase in the value of Pglyc. Unfertilized ova exhibited Pglyc values of 1 X 10(-5), 2 X 10(-6), and 5 X 10(-7) cm/min at approximately 20 degrees C, and approximately 12 degrees C, and approximately 3 degrees C, respectively. Fertilized ova exhibited Pglyc values of 3.4 X 10(-5), 1.3 X 10(-5), and 4.7 X 10(-6) cm/min at apoproximately 20 degrees C, 12 degrees C, and approximately 3 degrees C, respectively. The activation energy of glycerol permeation into fertilized and unfertilized ova was calculated to be 18.7 and 28.4 kcal/mol, respectively. Fertilization caused a rather dramatic increase in glycerol permeability, the major extent of which occurred within about 3 hours after fertilization. Pglyc was also measured at approximately 20 degrees C by isotope uptake. Glycerol uptake calculated indirectly from the volume changes of the ova agreed well with that measured directly by labeled glycerol uptake. It was also found that the zona pellucida fertilized and unfertilized ova did not detectably impede the permeation by glycerol.