A double-blind study of 24 weeks' duration was conducted to investigate the effects of ergoloid mesylates (Hydergine) on symptoms of senile mental deterioration. Fifty-eight residents of old people's homes were included in the trial. Thirty were treated with ergoloid mesylates and 28 with placebo, and the effects of treatment were determined by means of medical and psychological examinations. On the Sandoz Clinical Assessment Geriatric Scale, the group receiving ergoloid mesylates showed significant improvement in all items. The group receiving placebo showed slight deterioration. Psychological examination showed that no changes were observed for either group in quantitative psychometric test results, although qualitative aspects of performance such as attention and concentration did improve. There was a close correlation between improved cognitive function scores on the SCAG and improved evaluations of behavior during the psychological examinations. There were marked individual differences in the degrees of improvement.