An efficient treatment with psycho-drugs is essential for the initial therapy of acute schizophrenic psychoses. Also the outcome of the psychosis is largely dependent on the influence and the duration of the existing initial symptoms. Neuroleptic drugs raise the plasma prolactin level by blocking the dopamine receptors. So a valid parameter for the interference of the neuroleptics in the dopaminergic transmitter system is available. In 21 male patients the plasma prolactin levels were examined. The plasma prolactin base-lines were the same as in healthy persons. Infusions of 20 to 40 mg fluphenazine every three hours led to an average of fourfold increase of prolactin, which returned to the original level after 10 hours. The plasma prolactin level reached a plateau on the 7th day of treatment with fluphenazine dihydrochloride. Together with the stabilisation of the plasma prolactin level, which is reached more quickly with simultaneously administered fluphenazine decanoate, generally an improvement of the psychopathological condition takes place. In a comparable group of patients treated with ECT the prolactin level always returned to the initial point even after multiple administrations.