Chlorpyrifos is an organophosphorus (OP) anticholinesterase insecticide. Paraoxonase (PON1) is an enzyme found in liver and plasma that hydrolyzes a number of OP compounds. PON1 polymorphisms include a glutamine (Q)/arginine (R) substitution at position 192 (PON1(Q192R)) that affects hydrolysis of OP substrates, with the PON1(192Q) allotype hydrolyzing chlorpyrifos oxon less efficiently than the PON1(192R) allotype, a variation potentially important in determining susceptibility to chlorpyrifos. We studied 53 chlorpyrifos workers and 60 referents during 1 year and estimated chlorpyrifos exposure using industrial hygiene and employment records and excretion of the chlorpyrifos metabolite 3,5,6-trichloro-2-pyridinol (TCP). Plasma butyrylcholinesterase (BuChE) activity, which may by inhibited by chlorpyrifos exposure, was measured monthly. In addition, plasma samples were assayed for paraoxonase (PONase), diazoxonase (DZOase), and chlorpyrifosoxonase (CPOase) activity to determine PON1 status (inferred genotypes and their functional activity). Linear regression analyses modeled BuChE activity as a function of chlorpyrifos exposure and covariates. We postulated that the level of CPOase activity and the inferred PON1(192) genotype (together reflecting PON1 status) would differ between groups and that PON1 status would modify the models of chlorpyrifos exposure on BuChE activity. Chlorpyrifos workers and referents had a 100-fold difference in cumulative chlorpyrifos exposure. Contrary to our hypotheses, mean CPOase activity was similar in both groups (P=0.58) and PON1(192Q) showed a slight overrepresentation, not an underrepresentation, in the chlorpyrifos group compared with referents (PON1(192QQ), 51% chlorpyrifos, 40% referent; PON(192QR), 43% chlorpyrifos, 40% referent; PON(192RR), 6% chlorpyrifos, 20% referent, P=0.08). In our models, BuChE activity was significantly inversely associated with measures of interim chlorpyrifos exposure, but the biological effects of chlorpyrifos exposure on BuChE activity were not modified by PON1 inferred genotype or CPOase activity.