Evidence is accumulating which indicates that copper-deficient animals are prone to oxidative damage. To investigate this possibility further, we measured the production of breath ethane, a hydrocarbon by-product of lipid peroxidation, in copper-deficient rats. Male, weanling Sprague-Dawley rats were fed either a purified diet which was deficient in copper (CuD) or the same diet made sufficient with 5 ppm of copper (CuS). After 33 to 34 days the rats were placed individually in gastight metabolic cages through which ethane-free air or 100% O2 was passed. Expired ethane was absorbed onto cold, activated charcoal, liberated by heating, and measured by gas chromatography. Ethane production rates (pmoles/min/100 g +/- SD) were 3.3 +/- 0.8 (CuS-air), 4.3 +/- 1.4 (CuD-air), 8.3 +/- 2.5 (CuS-O2), and 12.2 +/- 4.3 (CuD-O2). Repeated measures analysis of variance indicated that both copper deficiency (P less than 0.01) and breathing 100% O2 (P less than 0.0001) enhanced ethane production, with no interaction between treatments. This finding complements previous evidence that increased lipid peroxidation occurs in copper-deficient rats.