A double-blind clinical trial was performed on 50 persons (25 men, 25 women, average age 77 years) suffering from dementias of medium level (DSM III, Category 1, ICD No. 299). All subjects were residents in an old age home. The patients were treated first for 2 weeks by placebo tablets. During this period, body composition parameters were determined and these data served as controls. This was then followed by an 8-week-long treatment with the nootropic drug, centrophenoxine (CPH), 2 g/day distributed in 2x2 tablets of Helfergin(500) (Promonta, Hamburg, F.R.G.), or placebo tablets of identical size. After 8 weeks the laboratory tests were repeated again. The randomization code for verum or placebo treatment was revealed only after completing the trial. Four dropouts occurred during the treatment period. The total body water, extracellular water volume and exchangable Na(+) were determined by means of radioisotope methods. Plasma volume (PV) was measured by indocyanine-green dilution test. Body weights were also recorded. From the above data further parameters could be calculated. Serum lipid data as well as several hormone levels were also checked in the blood samples. The main results were better performance in psychometric tests (Pék et al., 1989) and a significant increase of the average intracellular water content (2.2-2.5% by weight) in the verum-treated group. The rehydration of the intracellular mass due to CPH treatment is consistent with the OH(*.) free radical scavenger properties of CPH and the predictions of the membrane hypothesis of aging.