The metabolic stability of the acyl-CoA: cholesterol O -acyltransferase (ACAT) inhibitor N -(4-benzyloxy-3, 5-dimethoxycinnamoyl)- N ′-(2, 4-dimethylphenyl)piperazine (YIC-708-424) and its n -alkoxy derivatives containing an alkyl chain of 3 or 7 to 10 carbons, which exhibited different hypocholesterolemic activities, was investigated in vivo and in vitro in rats. After the oral administration of YIC-708-424 to rats at a dose of 5 mg/kg/d for 7 d, the parent compound was not detected in the blood. On the other hand, when the n -alkoxy derivatives were administered to rats, an increase in the alkyl chain length produced a progressive increase in the blood concentration of the parent compound. Both in the blood of rats administered YIC-708-424 and in the reaction mixture after the incubation of YIC-708-424 with rat hepatic 9000× g supernatants, an inactive major metabolite, N -(4-benzyloxy-3, 5-dimethoxycinnamoyl)- N ′-(4-carboxyl-2-methylphenyl)piperazine, was observed. The ratio of the maximum velocity to the apparent Michaelis–Menten constant ( V _max/ K _m) for the degradation of the n -propyloxy derivative in rat hepatic and intestinal microsomes was almost equivalent to that of YIC-708-424. On the other hand, an increase in the alkyl chain length of n -alkoxy derivatives produced a progressive decrease in V _max/ K _m for the degradation of these compounds. Additionally, the in vivo hypocholesterolemic activities of YIC-708-424 and its n -alkoxy derivatives were positively correlated with the blood concentration of the parent compound and were negatively correlated with their V _max/ K _m. These results suggest that the metabolic stability of ACAT inhibitors in the liver and intestinal epithelium, which are the major target organs of these compounds, has a strong influence on their pharmacological activities in vivo .