A model to study the effectiveness of potential chemopreventive agents that inhibit neoplastic process by different mechanisms has been used to test the efficacy of seven nitric oxide synthase (NOS) inhibitors. Five selective inducible NOS (iNOS) inhibitors: S-methyl isothiourea (S-MITU), S-2-aminoethyl isothiourea (S-2-AEITU), S-ethyl isothiourea (S-EITU), aminoguanidine (AG), 2-amino-4-methyl pyridine (2AMP), and two non selective general NOS inhibitors: L-N6-(1-iminoethyl) lysine (IEL) and Nω-nitro-L-arginine (NNLA), were tested for efficacy against a carcinogen, benzo[a]pyrene (B[a]P)-induced primary rat tracheal epithelial (RTE) cell transformation assay. RTE cells were treated with B[a]P alone or with five nontoxic concentrations of an NOS inhibitor and the resulting foci at the end of 30 days were scored for inhibition of transformation. The results indicate that all three isothiourea compounds inhibited B[a]Pinduced RTE foci in a dose-dependent manner. SAEITU was the most effective inhibitor with an IC50 (the molar concentration that inhibits transformation by 50%) of 9.1 μM and 100% inhibition at the highest dose tested (30 μM). However, both S-EITU and SMITU showed a maximum percent inhibition of 81% and 100% at 1 mM with an IC50 of 84 and 110 μM, respectively. 2-AMP did not show any dose-dependent response, but was highly effective (57% inhibition) at an intermediate dose of 30 μM and an IC50 of 25 μM. Similar to thiourea compounds, AG exhibited good dose-dependent inhibition with a maximum inhibition of 86% at 1 mM. NNLA and IEL were negative in this assay. Based on the IC50 values, NOS inhibitors were rated for efficacy from high to low as follows: S-2AEITU<2-AMP