Arylamine N-acetyltransferases (NAT) are important enzymes involved in the metabolic activation of aromatic and heterocyclic amines and inhibitors of NAT enzymes may be valuable as chemopreventive agents. Phytochemicals including cinnamic acid derivatives, various classes of flavonoids and coumarins were tested for the inhibitory activity on NAT1 and NAT2 from human liver and the human cholangiocarcinoma cell line: KMBC cells. Assays were performed using p-aminobenzoic acid and sulfamethazine as selective substrates for NAT1 and NAT2, respectively. NAT1 and NAT2 activities were present in liver cytosol. However, the KMBC cells showed only NAT1 activity. There was a marked difference in the ability of the test chemicals to inhibit NAT1 and NAT2. Caffeic acid, ferulic acid, gallic acid and EGCG inhibited NAT1 but not NAT2, whereas scopuletin and curcumin inhibited NAT2 but not NAT1. Quercetin, kaemferol and other flavonoids, except epicatechin and silymarin, inhibited both enzymes. The kinetics of inhibition of NAT1 by caffeic acid, EGCG and quercetin were of the non-competitive type, whereas that of NAT2 by quercetin, curcumin and kaemferol was also of the non-competitive type. The most potent inhibitor was quercetin, which has the inhibitory constants for NAT1 and NAT2 of 48.6 +/- 17.3 and 10.0 +/- 1.8 microM, respectively.