The effect of two different Ginkgo biloba extracts (GB1 and GB4) was studied in-vitro on cultured neurons exposed to oxidative stress caused by H2O2(50 micromol L(-1)) and FeSO4(100 micromol L(-1)). Only about 50% of the neurons were still viable at the end of the experiment (8 h) in control conditions, while the two extracts dose dependently increased the number of viable cells, in the concentration range 10-200 microg mL(-1). The two Ginkgo biloba extracts differed in their effect on hydroxyl-radical-scavenging capacity: GB1 and GB4 had an IC50 (50% inhibiting concentration) value of 78 microg mL(-1) and 186 microg mL(-1), respectively. However, both extracts inhibited apoptosis in cortical neurons after oxidative stress in-vitro. These observations make one suppose that different preparations of Ginkgo biloba have quantitatively different actions and outline the importance of the contribution of apoptosis prevention toward their neuroprotective action.