The benzoic acid derivatives of retinoic acid, often referred to as arotinoids, are synthetic retinoids that possess some of the properties of vitamin A. In general, these retinoids have more favorable therapeutic ratios, based on acute toxicity in adults, than all-trans-retinoic acid in cancer chemoprevention. In the present study, a single dose of (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8, 8-tetramethyl-2-naphthalenyl)-1-propen-1-yl]benzoic acid (Ro 13-7410; arotinoic acid), ethyl-(E)-4-[2-(5,6,7,8-tetrahydro-5,5,8,8- tetramethyl-2-naphthalenyl)-1-propen-1-yl]benzoate (Ro 13-6298; arotinoid ethyl ester), (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8, 8-tetramethyl-2-naphthalenyl)-1-propen-1-yl]phenylmethanol (Ro 13-8320; arotinoic methanol), or (E)-1,2,3,4-tetrahydro-1,1,4, 4-tetramethyl-6-[1-(4-methylphenyl)-1-propen-2-yl]naphthalene (Ro 13-9272; methyl arotinoid) was administered to pregnant Syrian golden hamsters during the early primitive streak stage of gestation. A significant increase in the numbers of litters containing one or more malformed offspring occurred at all doses of each retinoid studied. The types of malformations induced by oral arotinoid treatment were essentially identical to those found after maternal treatment with all-trans-retinoic acid or other teratogenic retinoids during the same gestational age. The results indicate that the alcohol congener was approximately 400 times more potent on a milligram per kilogram basis than all-trans-retinoic acid as a teratogen, and it was 70 times as embryolethal as all-trans-retinoic acid. The ethyl ester congener was 132 times, and the free acid 123 times, as embryolethal as all-trans-retinoic acid. On a molar basis, the arotinoic acid was at least 375 times as teratogenic as all-trans-retinoic acid and at least 140 times as teratogenic as etretinate in hamsters. Because the dose-response curves for arotinoids were significantly parallel to that for all-trans-retinoic acid, and because the spectrum of congenital defects induced by arotinoids was identical to that induced by all-trans-retinoic acid and other teratogenic retinoids, the mechanism of embryopathic action of these conformationally restricted retinoids was concluded to be similar to that of all-trans-retinoic acid.