Forty-eight male weanling Sprague-Dawley rats were randomly and equally divided into two dietary treatments (copper-deficient and adequate: 0.85 mg and 8 mg of Cu/kg diet). Deionized water and diet were provided ad libitum. After 5 weeks, the rats were fasted for 18 hours, anesthesized with sodium pentobarbitol and injected intravenously with glucose (1 g/kg body wt in a 50% wt/vol solution). Six rats from each treatment were killed by exsanguination at 0, 30, 60 and 120 minutes after glucose injection. Liver copper was measured by atomic absorption spectrophotometry. Reduction in liver copper content and elevation in heart weight confirmed that the rats fed the test diet were copper-deficient. Plasma glucose levels in copper-deficient rats were significantly higher at 30 and 60 minutes compared to controls. After 2 hours there were no significant differences between treatments. Plasma insulin levels measured by radioimmunoassay were significantly lower at 30 minutes, but higher at 60 and 120 minutes in rats fed the test diet as compared to controls. It would thus appear likely that copper deficiency interferes with normal glucose utilization.