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The effects of cadmium (Cd²⁺), mercury (Hg²⁺), lead (Pb²⁺), copper (Cu²⁺) and nickel (Ni²⁺) on the glutathione (GSH)–redox cycle were assessed in CHO-K1 by the neutral red uptake inhibition (NR) assay (NR_6.25, NR_12.5 and NR₂₅). Mercury proved to be the most and lead the least toxic of the metals tested. The effects on GSH content and intracellular specific activities of enzymes involved in the GSH–redox balance were measured after a 24-h exposure. Total GSH content increased significantly in cultures exposed to the lowest metal concentration assayed (NR_6.25), but fell to below control values when exposed to concentrations equivalent to NR₂₅. Oxidised glutathione content dropped significantly at NR_6.25, while somewhat higher values were obtained for cultures exposed to higher doses. Glutathione peroxidase (Gpx) activities were 1.2-, 1.5-, 1.6-, 2.0- and 2.5-fold higher than untreated controls for …