Novel and potent ACAT (acyl-CoA : cholesterol O-acyltransferase) inhibitors, (R)-N-2-(1, 3-benzodioxol-4-yl)heptyl-N'-2, 6-diisopropylphenylurea (2a, EAB-309), and its enantiomer 2b (EAB-310), were prepared from 4-(1, 3-benzodioxole)carbaldehyde (7) via optically active (R or S)-2-(1, 3-benzodioxol-4-yl)heptanoic acid (12a or 12b). Compound 2a showed potent inhibitory effects on ACATs in vitro, and lowered plasma cholesterol in vivo. The IC₅₀ value for inhibition of rat hepatic microsomal ACAT was 5 nM. The ED₃₀ values of hypolipidemic activities in hamster and rat models were 0.25 and 0.75 mg/kg p.o., respectively. The results indicate that 2a has potential to be a novel hypocholesterolemic and antiatherosclerotic agent. The activities of 2a in vitro and in vivo were only several times more potent than those of the enantiomer 2b. Modeling studies suggested that the three-dimensional structures of the two enantiomers are similar to each other.