Effect of an extract of Ginkgo biloba leaves (EGb) on oxidative metabolism was studied using rat brain neurons and 2',7'-dichlorofluorescin fluorescence. Ionomycin (100 nM to 1 microM), a Ca(2+)-ionophore, dose-dependently augmented the 2',7'-dichlorofluorescin fluorescence in the presence of external Ca2+, but not under the external Ca(2+)-free condition. Preincubation of neurons with EGb (3 micrograms/ml) greatly reduced the ionomycin-induced increase in 2',7'-dichlorofluorescin fluorescence. Results suggest that EGb may reduce the Ca(2+)-induced increase in the oxidative metabolism of brain neurons.