Pregnant mice were given to daily dose of 0, 0.18, or 9.0 mg Diazinon per kilogram body weight throughout gestation. Mothers of all dose groups gave birth to viable, overtly normal offspring. However, pups born to mothers receiving the higher dose of the organophosphate grew significantly slower than controls and remained significantly smaller at 1 month of age. Offspring of mothers receiving the lower dose apparently were unaffected, but systematic behavioral testing revealed subtle deviations from normal developmental ontogeny as shown by significant delays in the appearance of the contact placing reflex and of sexual maturity (descent of testes or vaginal opening). Mature offspring of mothers exposed to either dose of the pesticle displayed impaired endurance and coordination on rod cling and inclined plane tests of neuromuscular function. Offspring from the 9.0 mg/kg group, in addition, had slower running speeds in a Lashley III maze and less endurance in a swimming test. Brains obtained after sacrifice at 101 days of age revealed neuropathology in the forebrains of offspring born of mothers exposed to the higher dose. Despite functional impairments in offspring from the lower dose group, no corresponding brain pathology was observed by examination under the light microscope.