The present invention relates to a phenylpiperazine derivative represented by the general formula: ##STR1## wherein R.sub.1 stands for a lower alkyl group; R.sub.2 stands for a group represented by the general formula: ##STR2## (wherein R.sub.3, R.sub.4 and R.sub.5 each stand for a hydrogen atom or a lower alkyl group and Y stands for CH or N); and n stands for an integer of 1 to 6, and pharmaceutically acceptable acid addition salts thereof, which exhibit an .alpha..sub.1 -blocking activity and therefore are expected to be useful as a hypotensive drug.