In this experiment we investigated the effect of the antidepressant clomipramine (Anafranil) on twenty patients with Parkinson's disease. Ten of them had a relatively high and ten a relatively low 5-HIAA-concentration in the cerebrospinal fluid after loading with probenecid. The research-design was double-blind, half of the patients received a placebo and the other half clomipramine. The hypothesis that patients with a low 5-HIAA-concentration after administration of clomipramine should improve with respect to their mood, could not be sustained. The patients however, showed a significant improvement on the Hamilton-scale and on two psychological tests, which measure the social involvement of the patients. This improvement could be interpreted as the result of the attention which was given to them because of the experiment.