Fischer 344 rats were exposed by the nose-only inhalation route to chlorpyrifos vapors at concentrations of 0, 5.2, 10.3, or 20.6 ppb, 6 hr/day, 5 days/week for 13 weeks. The exposure concentrations were limited by the low vapor pressure of chlorpyrifos (theoretical maximum vapor concentration of 25 ppb at 25 degrees C). No treatment-related signs of toxicity or changes in body weights were detected during the course of the study. Urinalysis, hematology, clinical chemistry, organ weights, gross pathologic, and histopathologic evaluations were performed at the end of the study with no treatment-related effects observed. In addition, no differences from controls were noted in plasma, red blood cell, or brain cholinesterase activities. The results of this study indicate that the no-observed-effect level for chlorpyrifos vapor was the highest attainable concentration, 20.6 ppb, in male and female Fischer 344 rats.