The use of sequential measurements of plasma cholinesterase (ChE) activity for monitoring exposure to organophosphorus pesticides was investigated in the mallard (Anas platyrhynchos). At the onset of incubation, birds were assigned to treated (400 ppm methyl parathion in the diet), pair-fed (same daily food allotment on a g/kg/d basis as consumed by the treated bird of the pair), control or nonincubating control groups. Blood samples were collected weekly during egg-laying and every 3 days during incubation periods. Both plasma and brain samples were taken on day 24 of incubation. Plasma ChE was more variable than brain ChE among the birds and there were no significant correlations (p>0.05) between plasma and brain samples within treatment groups. Reduced food consumption during incubation did not affect plasma ChE activity (p=0.77). Birds that abandoned their nests had significantly increased (paired t=-2.39, d.f.=9, p<0.05) ChE activity at the time of abandonment. There were significant differences (p<0.01) ion plasma ChE activity between untreated birds, but within-bird variation was 2 and 11 times less than between-bird variation during egg-laying and incubation, respectively. Methyl parathion significantly (p<0.0001) reduced plasma ChE activity. Measurement of plasma ChE activity provides a noninvasive, nonlethal technique for assessing the exposure of animals to ChE inhibitors.