The present study was undertaken to measure the activities of several hepatic enzymes of regulatory importance in the pathways of lipogenesis and gluconeogenesis in rats fed diets marginally deficient in copper (1.2 micrograms Cu/g of diet) and containing either fructose, glucose, or starch as the carbohydrate sources. Although all copper-deficient rats exhibited the characteristic signs of copper deficiency, they were more pronounced in rats fed the diet containing fructose. Except for the activity of phosphoenolpyruvate carboxykinase which was unaffected either by copper deficiency or by the type of dietary carbohydrate, the hepatic activities of glucose-6-phosphate dehydrogenase, malic enzyme, L-alpha-glycerophosphate dehydrogenase and fructose 1,6-diphosphatase were unaffected by copper deficiency but were affected by the type of carbohydrate in the diet. Fructose produced the greatest increase in enzymatic activities, whereas starch produced the least activity and glucose induced an intermediate effect. These results indicate that the deleterious effects of a fructose diet deficient in copper on biochemical and physiological indices could not be due to an immediate metabolite of fructose. However, the involvement of a subsequent metabolite of fructose in the mechanism of copper utilization and/or requirement cannot be excluded.