Applications of 6 3 Ni in biological research are reviewed, with emphasis upon recent investigations of nickel metabolism and toxicology in experimental animals. The radiochemistry of 6 3Ni is summarized, including consideration of the preparation of certain 63Ni compounds (e.g. 63Ni(CO)q and 63Ni 3 s 2 ) that are of current interest in toxicology, teratology and cancer research. Practical guidance is given regarding the detection and quantitation of 6 3 Ni in biological materials by autoradiography and liquid scintillation spectrometry.