Half the young in each of four broods of 5-d-old mallards (Anas platyrhynchos) were gavaged orally with 4 mg/kg methyl parathion and released with their mother and untreated siblings in pond enclosures. Four control broods received corn oil only. Activity categories (swimming, preening, feeding, loafing) of hens and broods were noted every 15 min. and continuous observations were made of length of time to recovery or death (dosed ducklings) and of time spent in a behavior. There was a significant difference (X2 = 76.4, p <0.001) between behaviors of treated and control broods; treated broods mostly preened and loafed on land while control broods primarily fed and swam in open water. Hens remained with their broods and kept all ducklings together even when dosed young were too sick to move (for 90 to 155 min. posttreatment). Forty percent of dosed ducklings died the first day, whereas no control ducklings died. The results of the study indicate that methyl parathion can affect the brood-rearing phase of reproduction by direct mortality and through behavioral changes. (Copyright (c) 1988 SETAC.)