Two northern bobwhite (Colinus virginianus) reproduction tests were conducted concurrently to evaluate how the duration and time of initiation of methyl parathion exposure affected dose-response relationships of reproductive parameters. In the long-term exposure test, pairs of adult bobwhite were fed methyl parathion-amended diets (0,7,10,14,20 or 28 ppm) for 25 weeks: 10 weeks prior to the onset of laying, 6 weeks as they came into laying condition and 9 weeks during egg laying. In the short-term exposure test, quail received amended diets (0,10,14,20,28, or 40 ppm) for only three weeks during the egg laying period, followed by a three-week posttreatment period. Fourteen birds died in the long-term test, compared to two in the short-term test. Significant dose-related reductions in daily food consumption, egg production and the number of 14-d-old chicks were observed in both tests during the treatment periods. No dose-related effects on fertility, hatchability or chick weights were detected. In the long-term test there were dose-related decreases in adult body weight, brain and serum cholinesterase activity and female serum calcium concentrations. Cholinesterase and calcium were not measured in the short-term test. Eggshell weights were significantly decreased in both tests, but a dose-related decrease in eggshell strength and thickness was detected only in the short-term test.