Copper and zinc are readily determined in biological material by atomic absorption spectrophotometry. Two procedures are presented for the quantitative estimation of these metals in serum. One of these involves simple dilution and aspiration into the burner of the instrument. Suitably prepared standards must be used. The other method involves trichloroacetic acid preparation of the proteins prior to aspiration. With both methods satisfactory results in precision and recovery of added metals are obtained. Urinary copper determinations require chelation of the metal with ammonium pyrrolidino dithiocarbamate or some similar sequestering agent and concentration by extraction of the chelate into a suitable organic solvent. A similar procedure must be used in analyzing cow's milk for copper. Tissue analysis requires heating with boiling nitric acid and removal of the acid under reduced pressure prior to aspiration into the burner. It has been shown that procedural errors in such determinations are very much smaller than biological variation.