The action of a Ginkgo biloba extract (rökan, Tanakan, G.B.E.) in promoting blood flow has been demonstrated in several animal and human pharmacological studies. The aim of this present study was to estimate the action of the substance on the central nervous system in order to be able to assess its potential use as a therapeutic agent in geriatric patients with cerebral insufficiency. Quantitative pharmaco-EEG is the method of choice for studying the vigilance-promoting effects of a drug. It is incomparable for confirming the findings of behavioural and psychometric studies. 60 volunteers of either sex participated in the double-blind trial. They were aged 57-77 years and showed mental deterioration corresponding to their age. They were randomly divided into three experimental groups: 20 subjects received 3 X 40 mg/day G.B.E., 20 received 5 mg nicergoline and 20 received a placebo of similar appearance. The subjects underwent an extensive series of examinations before and 4, 8 and 12 weeks after the start of medication. Analysis of the EEG results for the whole group revealed no significant advantage of G.B.E. over the two reference substances with regard to vigilance. However, a subclassification of the subjects showed that the vigilance of those persons with a more unfavourable initial situation measured in the resting EEG could be clearly improved by chronic G.B.E. medication. This increase in vigilance was reflected at the behavioural level by an improvement of reaction times in the G.B.E. group by comparison with the reference substances.(ABSTRACT TRUNCATED AT 250 WORDS)