Human brain total superoxide dismutase activity (SOD) was assayed in the presence of increasing concentrations of neuroprotectives. Superoxide-dependent nitrobluetetrazolium (NBT) reduction served as control for direct radical interaction of these substances. High concentrations of the dopamimetic substances L-DOPA slightly and the monoamine oxidase B inhibitor selegiline more effectively inhibit SOD activity. The MAO-B inhibitor RO 16-6491 (N-(2-aminoethyl)-4-chlorobenzamide hydrochloride) has no effect on SOD enzyme activity. Reduced glutathione stimulates SOD activity. Moreover it exhibits slight activity in scavenging radicals in vitro. Oxidized glutathione and vitamin E are unable to do so. Ascorbic acid mimics the activity of reduced glutathione, but directly interacts with NBT reduction. Thioctic acid shows no effect on SOD activity but stimulates superoxide-dependent NBT reduction. The Ginkgo biloba extract EGb 761 is highly active in inhibiting superoxide-dependent NBT reduction as well as SOD activity.