The potent heterocyclic food mutagen IQ is carcinogenic in the CDF1 mouse, affecting the liver, lung and forestomach. Using 32P-postlabelling methods we have isolated up to five IQ-DNA adducts from both target and non-target organs after oral administration of IQ. Up to four additional non-specific adducts, found when the 32P-postlabelling assay was run under intensification conditions, could be attributed to the use of a certain brand of microcentrifuge tube in the assay. CLA is a mixture of heat-generated derivatives of linoleic acid, each with a conjugated double bond system, that has chemopreventive properties in rodents. To examine the effect of CLA on the formation of IQ-DNA adducts in CDF1 mice, we administered CLA by gavage every other day for 45 days, followed by a single oral dose (50 mg/kg) of IQ. Tissues collected 24 h later were analysed for IQ-DNA adducts by 32P-postlabelling. Compared to controls, CLA treatment caused inhibition of adduct formation in the liver, lung, large intestine and kidney. In the kidneys of females, CLA treatment inhibited IQ-DNA adduct formation almost completely (95.2%). Male F344 rats were fed a control diet or an isocaloric diet containing 20% menhaden oil (MO), a rich source of omega-3 fatty acids, for six weeks, then given a single oral dose of IQ. Analysis for IQ-DNA adducts by 32P-postlabelling one or six days later revealed that on day 1 the MO diet caused a 6-7 fold decrease in adduct formation in the liver and an up to 2-fold decrease in both the small and large intestine.(ABSTRACT TRUNCATED AT 250 WORDS)