Dietary administration of the monocyclic monoterpenoid d-limonene causes complete regression of both dimethylbenz[alpha]anthracene- and N-nitroso-N-methylurea-induced rat mammary carcinomas. Carcinomas regress when limonene is added to the diet either when the tumor is small and still capable of spontaneously regressing or when it is large and progressed beyond the stage when it is susceptible to spontaneous regression. The limonene dose-tumor regression response relationship is steep. Significant regressions are not observed at 5% dietary levels, while a majority of tumors completely regress above a 7.5% dietary level. Limonene appears to act in a cytostatic fashion. Its removal from the diet results in a significant number of tumor recurrences. Regressing tumors have a unique histopathological appearance that is not associated with gross cytotoxicity, immune cell involvement, or apoptosis. Preliminary analysis suggests a remodeling/redifferentiation event underlying regression. The underlying mechanism of action of limonene in causing tumor regression is unknown. However, it should be noted that limonene can selectively inhibit the isoprenylation of small G proteins. Monoterpenoids such as limonene represent a novel class of anticancer drugs with the potential to cause tumor regressions with limited toxicity.