The first aim of this study was to evaluate the degree of organophosphorus ( OP) pesticides' exposure in viniculture and tobacco production workers via physical examination ( neurology and general health aspect) and analysis of paraoxonase ( PON1) and butyrylcholinesterase ( BuChE) activities. The second aim was to investigate if PON1 polymorphism plays any role in long-term OP exposure. A total of 93 farmers who work as applicators in agriculture were studied. The data were evaluated according to agricultural type, and although the total exposure time was similar in both areas, BuChE and PON1 activities of farmers who work in tobacco production were lower. Overall, BuChE and PON1 activities showed a depletion in the farmer group compared to age-matched controls. When the farmers were categorized according to the number of their symptoms, the BuChE activities of farmers who had two or more symptoms were found to be depleted ( n = 43, 2948 +/- 756) compared to farmers who had one or no symptoms ( n = 37, 3356 +/- 659). Allele frequencies of patients and controls for positions 55 and 192 were similar to Turkish population data and there was no association between the allele polymorphism and symptoms/signs of long-term exposure. Our results indicate that there is an important inhibition of PON1 activity in chronic OP poisoning, and this together with BuChE activity might well be used as a reliable index of chronic exposure to OP.