A series of reentry studies were made in 1971 through 1973 to determine the safe procedures to follow when using organophosphorus compounds on cotton fields to control pests. Human volunteers entered the fields, spending 30 minutes there, at 0, 12, 24, 48, and 72 hours following application of the pesticides. Exposure times were then extended to 5 hours to more nearly simulate the exposure of cotton insect checkers during an 8 hour working day. No objective or symptomatic indications of organophosphate poisoning occurred to any subject. Blood samples and urine samples were analyzed both before, during and after field exposures and analyzed for the parent compounds by flame photometric gas chromatography. Small depressions of red blood cell cholinesterase (ChE) activity were noted in subjects exposed at 12 hours to residues of methyl- parathion (298000), at 48 hours to ethyl-parathion (56382) residues in wet cotton, and at 24 and 48 hours to Azodrin (6923224). Plasma ChE was lower only in those exposed to Azodrin. Urinary excretion of PNP correlated with blood serum levels of either parathion and with leaf surface residues. Respiratory exposure was not a significant factor.