7,12-Dimethylbenz[a]anthracene (7,12DMBA) is a member of the polycyclic aromatic hydrocarbons with a severe carcinogenic effect. In this study, nitrate levels and ADA (Adenosine deaminase) activity in the liver homogenates of mice were measured and the effect of free radicals induced by 7,12-DMBA on inducible nitric oxide synthase (iNOS) and ADA activity were investigated. Antioxidant effects of melatonin were also compared. Three groups of mice were included in the study. The first served as control, the second was treated only with 7,12-DMBA and the third was treated with 7,12-DMBA+melatonin. Spectrophotometric methods were used at all measurements. Data were analyzed using Kruskal-Wallis Variance Analysis Test and Mann-Whitney U Test that were applied to the groups. The nitrate concentrations of mouse liver were as follows: 4.98 +/- 0.63 mumol/L in the control group (n = 10), 8.23 +/- 1.58 mumol/L (higher than control group, p < 0.05) in the 7,12-DMBA-treated group (n = 10), and 6.43 +/- 0.57 mumol/L (lower than 7,12-DMBA-treated group, p < 0.05) in the 7,12-DMBA+melatonintreated group (n = 10). Liver ADA activities were measured to be 4.14 +/- 0.674 U/L in the control group, 6.25 +/- 1.261 U/L (higher than control group, p < 0.05) in the 7,12-DMBA-treated group, and 4.93 +/- 0.916 U/L (lower than 7,12-DMBA-treated group, p < 0.05) in the 7,12-DMBA+melatonin-treated group. Differences between free nitrite levels were no significantly. Results demonstrated that nitrate levels and ADA activities were increased by means of free radicals induced by 7,12-DMBA. Melatonin inhibited the 7,12DMBA induced increase that was observed in the activities of ADA enzyme and nitrate levels. It is concluded that determination of ADA activity and nitrate levels can be useful in the assesment of liver damage caused by toxic chemicals. (C) 2002 Wiley Periodicals, Inc.