Abstract Sodium periodate in concentration 6.10 −5–9.10 −5 N has an activating effect on the eggs of Psammechinus miliaris. A membrane elevation occurred and cleavage started. It did not proceed beyond the 2–8 cell stage. The eggs activated subsequent to the periodate treatment exhibited the angular plasmolysis characteristic of fertilized eggs when immersed in a hypertonic medium. Even non activated periodate treated eggs behave differently from non pretreated eggs. The latter are often somewhat elongated in one direction and show a fine wrinkling on the surface. The former are spherical with a smooth surface. This behaviour indicates that the eggs have taken a step towards activation. Eggs activated subsequent to periodate treatment are as strongly stained with methylene blue as are fertilized eggs. In contrast the non activated pretreated or the non pretreated unfertilized eggs are very weakly or not at all stained with methylene blue. Iodate of corresponding concentrations has a much lower activating effect than sodium periodate. The activation of the eggs by periodate occurred even in absence of Ca. After transfer to Ca-containing medium the eggs underwent a contraction and the membrane became much more refractile.