In this paper are reported the effects of organophosphate pesticide exposure. Agricultural workers were compared with an age- and sex-matched control group selected from the general population. A significant difference was detected (Student's t = 1.99, p = .05) between the median activity levels of serum cholinesterase in agricultural workers at the end of the work day (mean 4.44, 95% CI = 4.12-4.77), compared with those in the control group (mean = 4.85, 95% CI = 4.57-5.13). The decrease in cholinesterase activity, measured at the beginning (mean = 4.70, 95% CI = 4.38-5.02) and end of the work day, was very significant (paired t = 2.70, p = .01). An interaction was identified between the type of work and the worker's age. Younger workers tended to perform more dangerous activities, and they experienced a greater decrease in cholinesterase activity. This study indicates that subclinical intoxication of agricultural workers, by pesticides, is occurring in Mexico and indicates the need for primary prevention programs that address occupational contact with such substances.