1. d-Limonene, a monoterpenoid constituent of citrus fruit oil, blocks tumour induction by chemical carcinogens in laboratory animals, apparently by preventing bioactivation of procarcinogens and by enhancing conjugation of proximal carcinogenic metabolites. 2. Inhibitory effects of d-limonene were measured in vitro using cytochrome P450 isoform-specific substrates. d-Limonene inhibited p-nitrophenol hydroxylase (pNP) activity in vitro in liver microsomes from acetone-, phenobarbital (PB)- and naphthoflavone (BNF)-treated mouse, and 7-ethoxyresorufin O-deethylase (EROD) activity in microsomes from PB- and BNF-treated mouse. p-Nitrophenol and ethoxyresorufin are substrates for cytochromes P2E1 and P1A1, respectively. No inhibition of benzphetamine (BNZP) or aminopyrine (AP) demethylases by d-limonene was observed. 3. EROD, BNZP and AP activities in liver microsomes were increased 18 h after i.p. administration of d-limonene to acetone-induced mouse, while pNP activity was unchanged. The immunodetectable protein level of cytochrome P2B1 in non-acetone treated mouse was increased 18h after d-limonene, with no differences in P2E1 or P1A1. 4. Acute d-limonene did not protect against paracetamol (acetaminophen)-induced depletion of liver reduced glutathione (GSH). A prolonged paracetamol challenge (0-6% diet for 10 days) elevated liver cytosolic GSH-S-transferase activity (GST) two-fold and decreased liver GSH to 46% of control values. Dietary d-limonene (1-0% diet for 10 days) maintained liver GSH concentrations at 92% of control values in the paracetamol-challenged mouse without altering GST activity. d-Limonene also increased liver GSH concentration (23% in mouse fed 1-0% d-limonene alone.