Twenty-one adult volunteers (aged 27-32 years), who had been living in Mexico City for four continuous months (physicians working as fellows) were studied the first and sixteenth week of their stay in order to learn the effects of the pollutants contained in Mexico City's atmosphere on some serum biochemical parameters. The activity of serum superoxide dismutase (SOD) decreased after 16 weeks in comparison with the values obtained the first week (109.6 to 56.9 mU/mg protein; 50% less). In contrast, the inhibitory capacity of serum vs. induced in vitro lipoperoxidation increased in relation to the length of stay (22%). The serum levels of thiobarbituric-reactive material also decreased in almost 30% (from 6.10 to 4.12 nmol). The other lipoperoxides measured were unchanged (chromolipids and diene conjugation). We propose that this may be as a result of the adaptative capacity of the human organism, within a pollutant atmosphere in which the ozone levels might participate in a decrease of SOD activity during chronic exposure, to air pollution.