We describe a technique for isolating the nucleus of the giant amphibian oocyte under paraffin oil. The method precludes the losses of small solutes and proteins that accompany isolation of nuclei into aqueous media. An individual oocyte is blotted, placed under oil, punctured near the animal pole and then squeezed to gently extrude the nucleus into the oil, thereby avoiding exposure to any aqueous environment. Light and electron microscopy of the oil-isolated nucleus demonstrate that its in vivo morphology is preserved. We also describe techniques that facilitate the study of nuclear functions under oil. Oil-isolated oocyte nuclei retain many in vivo functions for several hours, including size-selective envelope permeability, RNA synthesis and the ability to break down in response to cdc2/cyclin meiotic maturation promoting factor.