Phenylpiperazine derivatives of the formula: ##STR1## wherein R.sub.1 is a hydrogen atom or a straight- or branched-chain saturated or unsaturated alkyl group having 1 to 8 carbon atoms which may be substituted at any position by a hydroxyl group, a carboxyl group, a lower alkoxycarbonyl group or an oxo group; and R.sub.2 is a hydrogen atom, a halogen atom, a lower alkyl group or a sulfamoyl group, and a process for producing the same are disclosed. The derivatives of the formula have alpha blocking and serotonin antagonizing activities and are useful as a drug.