The nicotine and oxidants from cigarette smoke, could generate oxidative stress in human body. In our study, we determined malondialdehyde and free thiol groups in smokers plasma, the experiment was effectuated to blood donors, smokers (20 persons) and non-smokers (12 persons). The determinations was effectuated with 2-thiobarbituric acid for malondialdehyde and with 5,5'-DTNB for thiol groups. The results was expressed in mmol/L for malondialdehyde and mmol/ml for free thiol groups. Smokers presented higher values for malondialdehyde (1.519 +/- 0.683), than non-smokers (1.362 +/- 0.664). The free thiol groups values was decreased at smokers (0.502 +/- 0.136), than non-smokers (0.517 +/- 0.120). Statistically, insignificant differences (p > 0.05) between smokers and non-smokers, for both parameters, could be determined by the small number of cigarettes smoked (12-15 cigaretts/day), also by the alcohol, because all persons drunk occasionally alcohol.