Current methods for evaluating the toxicity of metal compounds in experimental animals include assessments of LD50, body and organ weights, assessments of body-burdens, organ-burdens, and toxicokinetics, histopathological and ultrastructural examinations, hematological, immunological, endocrine, reproductive, mutagenic, teratogenic, and carcinogenic tests, neurophysiological and behavioral observations, and, of especial relevance to this Conference, various biochemical techniques. The biochemical indices of metal toxicity that are considered in this paper include ( a) quantitative measurements of metallothionein (MT), glutathione (GSH), and related thiol-compounds in tissues and body fluids; ( b) induction of microsomal heme oxygenase activity in liver, kidney, and other organs; ( c) determinations of N-acetyl-glucosaminidase (NAG), beta-2 microglobulin, total protein, and amino acid excretions in urine, and ( d) HPLC-analysis of metal binding to macromolecualr ligands in cytosol and in detergent-solubilized organelles from target tissues. Reference values for these biochemical indices are compiled, based upon analyses of tissues and body fluids from untreated rats, and illustrative abnormal results are presented, derived from studies of the acute toxicity of nickel compounds in rats. For examples, metallothionein concentrations are increased 2 to 4 fold, and heme oxygenase activity is increased 3 to 10 fold in kidney and liver of rats killed 17 h after sc injection of NiCl 2 (0.5 mmol/kg). Urinary excretions of NAG and total proteins are increased 2 to 4 fold at 48 h after ip injection of NiCl 2 (0.1 mmol/kg). Under the same conditions, urinary excretions of certain amino acids (Lys, Leu, Ileu, Val) are increased 15 to 23 fold. Gel-filtration chromatography (HPLC on Toyo-Soda SW2000 and SW3000 columns) of renal cytosol and detergent-solubilized renal microsomes of rats killed 1 h after im injection of 63NiCl 2 (0.03 mmol/kg) reveal five distinct macromolecular 63Ni-binding components, with molecular weights from 6,000 to 600,000. A rich array of biochemical tests is available to elucidate toxic effects of metals on the kidney, liver, and other organs. Based upon the author's experience with nickel, measurements of metallothionein, heme oxygenase, and metal-binding ligands in tissue homogenates, and assays of N-acetylglucosaminidase, total proteins, and amino acids in body fluids are particularly sensitive, specific, and significant as biochemical indices of metal toxicity.