Abstract:Flupenthixol, a thioxanthene derivative may exist in two isomeric forms, α and β. Flupenthixol contains 45‐55 % α‐flupenthixol. The pharmacological effects of flupenthixol, α‐ and β‐flupenthixol have been compared with those of clopenthixol, chlorprothixene, fluphenazine, perphenazine, chlorpromazine and haloperidol. In most pharmacological screening tests α‐flupenthixol was equipotent with fluphenazine. β‐Flupenthixol showed very low pharmacological activity. As expected the potency of flupenthixol was about one half that of α‐flupenthixol. Flupenthixol was considerably more potent than clopenthixol and chlorprothixene in the inhibition of conditioned avoidance response. The influence on temperature regulation and the peripheral α‐adrenolytic effect was considerably weaker than