Plasma levels of penfluridol and thiothixene were studied after 4 weeks treatment in a double-blind controlled trial of 47 patients suffering from chronic schizophrenic syndromes. There was found a tenfold variation in plasma levels for penfluridol, and about a twentyfold variation for thiothixene. For penfluridol, a significant correlation between dosage and plasma level and also between dosage and changes in psychopathology as regards factor 5 in the Mårtens & Jonsson S scale which comprises the items most characteristic of a schizophrenic syndrome, was found. For thiothixene, a significant correlation between plasma levels and changes in factor 5 was found. A gas-chromatographic method for penfluridol is also described.