Twelve 4-substituted semicarbazone derivatives of o- and p-chloro- as well as 2,6-dichlorobenzaldehyde were synthesized and investigated for antihypertensive activity in spontaneously hypertensive rats. Several of the compounds synthesized (viz. 1, 6, 7, and 15) exhibited potent antihypertensive effects when orally administered. The same compounds were not hypotensive in the normotensive dog.