Chlorpyrifos is an extensively used organophosphate insecticide with numerous agricultural crop and urban pest control uses. Dow AgroSciences, one of the manufacturers of chlorpyrifos, convened a panel of toxicology and medical experts to consider the available scientific literature (both published papers and unpublished reports from Dow AgroSciences) on chlorpyrifos with respect to determining the acute and chronic toxicological reference doses (RfD) for chlorpyrifos. The most sensitive effect observed in the body of animal and human studies on chlorpyrifos is the inhibition of the various cholinesterases. In animal studies chlorpyrifos is not carcinogenic, teratogenic, or mutagenic. Reproductive toxicity studies indicate some effects on postnatal survival, but these effects were seen at doses higher than maternal toxic dose levels. There are no clinical signs of cholinergic toxicity below 70% inhibition of brain cholinesterase. Cognitive or behavioral defects are not observed until substantial brain cholinesterase inhibition occurs. There are no indications that chlorpyrifos caused delayed neurotoxicity at dose levels below twice the oral LD50. The panel members stated that plasma cholinesterase was an inappropriate endpoint for the RfD. They recommended that brain, erythrocyte cholinesterase depression and clinical signs were appropriate as endpoints for the RfD. Plasma cholinesterase should be used only as an indicator of exposure. After a thorough review of the experimental animal literature, it was determined that the chlorpyrifos repeated-exposure RfD based on application of a 100-fold uncertainty factor on the no observed adverse effects level (NOAEL) for brain cholinesterase inhibition or on a 10-fold uncertainty factor on the NOAEL for erythrocyte cholinesterase inhibition is 0.01 mg/kg/d. Based on the human volunteer studies, which indicate a repeated-dose NOAEL of 0.1 mg/kg/d for erythrocyte cholinesterase inhibition, and then using a 10-fold uncertainty factor, the RfD is again 0.01 mg/kg/d. In this human volunteer study on d 9 (last day) of administration, 1 individual in the 0.1 mg/kg/day dose group was removed from the study due to a "cold" (runny nose, blurred vision, and a feeling of faintness). He was asymptomatic 12 h later. To some degree this diagnosis is supported by the hematology, since lymphocyte counts were reduced and neutrophil counts were increased markedly, indicating a possible inflammatory reaction on d 8 of dosing, clearing by posttreatment d 5. In the absence of any indication of erythrocyte cholinesterase inhibition and with plasma cholinesterase inhibition being greater in two of the four other individuals treated at the same dose level, these signs and symptoms are unlikely to have been induced by cholinesterase inhibition. The panel concluded that the symptoms this individual displayed were not an appropriate endpoint for the RfD. The single-exposure reference dose can be based on the human data. No erythrocyte cholinesterase inhibition or clinical signs of toxicity were observed after a single oral dose of 0.5 mg/kg. Utilizing a 10-fold uncertainty factor, the single-dose RfD is 0.05 mg/kg.