Zona drilled hamster and human oocytes were examined by scanning electron microscope. In the hamster mechanical drilling by micropipette created a beautiful hole mimicking sperm penetration hole left on the zona. Chemical drilling with acidic solusion (PH 2.2) made a round, straight hole with the size corresponding to the micropipette used. In the human oocytes, mechanical drilling formed a round, somewhat irregular, micropipette sized hole. Neither the zona surface, nor the microvilli were damadged. On the contrary, drilling with ascidic solusion resulted in the formation of a large, V-shaped hole, reflecting the different solubility properties of human zona pellucida. The microvilli on the plasma membrane were apparently affected. These results indicate that a simple drilling by micropipette is enough to create a hole sufficient for sperm passage, and that acidic solusion is not recommended for human zona drilling because of potential damadge to the vitellus.