Twenty-three naturally occurring and synthetic alkenylbenzene derivatives structurally related to the hepatocarcinogen safrole (1-allyl-3,4-methylenedioxybenzene) were assayed for their hepatocarcinogenicity in mice. Some of these compounds (safrole, estragole, eugenol, anethole, methyleugenol, myristicin, elemicin, and dill and parsley apiols) may be ingested in very small amounts by human as natural components of certain spices, essential oils, or vegetables. Estragole (1-allyl-4-methoxybenzene) and its proximate carcinogenic metabolite 1'-hydroxyestragole, previously shown to induce hepatic tumors when administered to male CD-1 mice only during the preweaning period, also induced hepatic tumors on administration for 12 months in the diet of female CD-1 mice. Eugenol (1-allyl-4-hydroxy-3-methoxybenzene) and anethole (trans-4-methoxy-1-propenylbenzene) were inactive in this assay; they were also inactive when administered i.p. during the preweaning period at total doses of up to 9.45 mumol/mouse to male CD-1 or C57BL/6 x C3H F1 (hereafter called B6C3F1) mice. Methyleugenol (1-ally-3,4-dimethoxybenzene) and its 1'-hydroxy metabolite had activities similar to those of estragole and its 1'-hydroxy metabolite for the induction of hepatic tumors in male B6C3F1 mice treated prior to weaning; 1-allyl-1'-hydroxy-4-methoxynaphthalene was somewhat less active. At the levels tested, myristicin (1-allyl-5-methoxy-3,4-methylenedioxybenzene), elemicin (1-allyl-3,4,5-trimethoxybenzene) and its 1'-hydroxy metabolite, dill apiol (1-allyl-2,3-dimethoxy-4,5-methylenedioxybenzene), parsley apiol (1-allyl-2,5-dimethoxy-3,4-methylenedioxybenzene), 1'-hydroxyallybenzene, 3'-hydroxyanethole, and benzyl and anisyl alcohols had no detectable activity for the initiation of hepatic tumors on administration to male mice prior to weaning. The acetylenic derivative 1'-hydroxy-2',3'-dehydroestragole was much more active than either 1'-hydroxysafrole or 1'-hydroxyestragole when administered to preweanling mice. The 2',3'-oxides of safrole, estragole, eugenol, and 1'-hydroxysafrole, which are metabolites of these alkenylbenzenes, had little or no activity in this test. The 2',3'-oxides of safrole and estragole and their 1'-hydroxy derivatives likewise had little or no activity for the induction of lung adenomas in female A/J mice or for the induction of tumors on repetitive injections s.c. in male Fischer rats. However, the 2',3'-oxides of safrole, estragole, eugenol, 1'-hydroxysafrole, and 1'-hydroxyestragole, when administered topically to female CD-1 mice at relatively high doses, initiated benign skin tumors that could be promoted with croton oil.