Parenteral administration of nickel chloride (NiCl2) to rats enhanced lipid peroxidation in liver, kidney, and lung (but not in brain, heart, spleen, or testis), as measured by the thiobarbituric acid reaction for malondialdehyde (MDA) and related chromogens in fresh tissue homogenates. After sc injection of NiCl2 (0.75 mmol per kg body wt), MDA concentrations in liver and kidney became significantly increased by nine h and reached peak values at 48 h. For example, in nine rats killed 48 h after the NiCl2 injection, hepatic MDA concentrations averaged 2.5 +/- 1.0 mumol per g dry wt (P less than 0.001 versus 0.5 +/- 0.3 mumol per g in 30 controls). Dose-effect relationships for lipid peroxidation in liver and kidney were observed with NiCl2 dosages ranging from 0.12 to 0.75 mmol per kg, sc. Intrarenal administration of a carcinogenic nickel compound, nickel subsulfide (Ni3S2, 0.36 mmol per kg body wt), did not affect MDA concentrations in the injected kidneys of rats killed one to 20 days post-injection. The results of this study implicate lipid peroxidation as a molecular mechanism for cell injury in acute NiCl2 poisoning, but they do not furnish any evidence that lipid peroxidation is involved in the initiation of nickel carcinogenesis.