Induction of the carcinogen-metabolizing enzyme cytochrome P4501A1 (CYP1A1) is a key step in the development of tobacco-related cancers. To determine if marijuana smoke activates CYP1A1, a murine hepatoma cell line expressing an inducible CYP1A1 gene (Hepa-1) was exposed in vitro to tar extracts prepared from either tobacco, marijuana, or placebo marijuana cigarettes. Marijuana tar induced higher levels of CYP1A1 messenger RNA (mRNA) than did tobacco tar, yet resulted in much lower CYP1A1 enzyme activity. These differences between marijuana and tobacco were primarily due to Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the psychoactive component of marijuana. Here we show that Delta(9)-THC acts through the aryl hydrocarbon receptor complex to activate transcription of CYP1A1. A 2-microg/ml concentration of Delta(9)-THC produced an average 2.5-fold induction of CYP1A1 mRNA, whereas a 10- microg/ml concentration of Delta(9)-THC produced a 4.3-fold induction. No induction was observed in Hepa-1 mutants lacking functional aryl-hydrocarbon receptor or aryl-hydrocarbon receptor nuclear translocator genes. At the same time, Delta(9)-THC competitively inhibited the CYP1A1 enzyme, reducing its ability to metabolize other substrates. Spiking tobacco tar with Delta(9)-THC resulted in a dose-dependent decrease in the ability to generate CYP1A1 enzyme activity as measured by the ethoxyresorufin-o-deethylase (EROD) assay. This inhibitory effect was confirmed by Michaelis-Menton kinetic analyses using recombinant human CYP1A1 enzyme expressed in insect microsomes. This complex regulation of CYP1A1 by marijuana smoke and the Delta(9)-THC that it contains has implications for the role of marijuana as a cancer risk factor.