Abstract: The effects of triflupromazine sulphoxide on blood pressure and on vasopressor responses to adrenaline and noradrenaline in rats were studied and compared with those of chlorpromazine and triflupromazine. For this purpose, the phenothiazines were ad- ministered in doses which produced the same degree of sedative action in mice. The results revealed that triflupromazine sulphoxide given in doses 2.4-2.6 times the other two drugs elicited only weak depressor action and practically no "adrenaline reversal" effect, while triflupromazine was somewhat similar to chlorpromazine in its strong depres- sor action and prominent "adrenaline reversal" effect. All the three drugs decreased the vasopressor effect of noradrenaline but only chlorpromazine yielded results with statistical significance. The relation between the oxidation on the S-atom and the depressor effect of pheno- thiazines was discussed.