A series of N-phenyl-6,11-dihydrodibenz[b,e]oxepin-11-carboxamides and related derivatives were prepared on the basis of structures of the reported inhibitors of acyl-CoA:cholesterol acyltransferase (ACAT). These compounds were tested for their ability to inhibit ACAT (liver microsomes from cholesterol-fed rabbits) in vitro and to decrease serum total cholesterol in cholesterol-fed golden hamsters in vivo. The structure-activity relationships in vitro were as follows. Substitution at positions 2 and 6 in the anilide resulted in potent inhibitory activity, and the potency increased with increasing size of the substituents, with maximum potency being obtained with a 2,6-diisopropyl substitution. The position of the substituent on the dibenz[b,e]oxepin ring system influenced the activity, and substitution at position 2 was critical for potent activity. The electronic effect of the substituent at position 2 does not influence activity, but bulkiness seems to be a significant factor. The lipophilicity of the compounds also plays an important role in determining ACAT inhibitory activity. Among the compounds tested, 2-bromo-N-(2,6-diisopropylphenyl)-6,11-dihydrodibenz-[b,e]++ +oxepin-11- carboxamide (33, KF17828) showed significant in vitro activity (rabbit liver microsomes IC50 = 23 nM) and the most potent in vivo activity (complete reduction in elevated serum total cholesterol levels at a dose of 10 mg/kg in hamsters).