The clinical effects of thiothixene and its action upon the HNA have been investigated in a sample of 22 chronic schizophrenic patients (mean duration of the disease 9.6 years). In three patients there was a good, and in nine patients there was a moderate, clinical improvement. In HNA there was a marked improvement of the stability of excitatory process, of the equilibrium of excitatory and inhibitory processes and of internal inhibition. Some of the HNA parameters reflecting the intensity of transmarginal inhibition were found to be of prognostic value.