Blood plasma cholinesterase (ChE) activity is a sensitive indicator of exposure to organophosphorus and carbamate insecticides. Effects of sex and storage of samples were studied as sources of variability by treating breeding Japanese quail (Coturnix japonica) with 3 mg of dicrotophos or carbofuran per kg of body weight and comparing blood plasma ChE activities for samples collected at 1 hr postdosage and assayed fresh, after 1 and 2 days of refrigeration (4 C), and after 1, 7 and 28 days of freezing(−25 C). ChE activity of fresh control plasma was 34% (P < 0.01) higher in males than females. Male ChE activity remained essentially unchanged during storage while female ChE activity increased (P < 0.05) gradually over time under both storage conditions. In contrast, when plasma ChE activity was inhibited by either antiChE, male plasma ChE activity was depressed further than female ChE (P < 0.01) and remained constant during storage while female ChE activity continued to decrease (P < 0.05). These divergent effects of exposure to antiChE compounds and sample storage indicate extreme care should be exercised in use of blood plasma for evaluation of antiChE exposure in wild birds.