1,4-Dihydro-2,6-dimethyl-4-(3-nitrophenyl)-3,5-pyridinedicarboxylic++ + acid methyl 1-(phenylmethyl)-3-piperidinyl ester 1, a highly potent calcium antagonist, was separated into stereo- and optical isomers to investigate the differences of antihypertensive activities between them. Fractional crystallization of the hydrochlorides of 1 gave alpha- and beta-diastereoisomers. The alpha-isomer (benidipine hydrochloride, KW-3049) showed very strong hypotensive effect, but little activity was observed in the beta-isomer. From optically resolved 1,4-dihydro-5-methoxycarbonyl-2,6-dimethyl-4-(3-nitrophenyl)-3- pyridinecarboxylic acids 2, and 1-benzyl-3-piperidinols 3, four optical isomers of 1 were synthesized, and their absolute configurations were deduced. The hypotensive activity of (+)-alpha, namely (S)-(S)-1, was 30 to 100 times stronger than that of (-)-alpha in intravenously administered spontaneously hypertensive rats.