Age-related differences in the acute neurotoxicity of cholinesterase (ChE)-inhibiting pesticides have been
well-studied for a few organophosphates, but not for many others. In this study, we directly compared doseresponses
using brain and red blood cell (RBC) ChE measurements, along with motor activity, for mevinphos,
monocrotophos, dicrotophos, and phosphamidon. Long–Evans hooded male rats were tested as adults and at
postnatal day (PND) 17; PND11 pups were also tested with dicrotophos only. All chemicals were administered
via oral gavage and tests were conducted at times intended to span peak behavioral and ChE effects. All OPs
tested produced a rapid onset and recovery from the behavioral effects. There were age-related differences in
the inhibition of brain, but not necessarily RBC, ChE. Mevinphos was clearly more toxic, up to 4-fold, to the
young rat. On the other hand, monocrotophos, dicrotophos, and phosphamidon were somewhat more toxic to
the young rat, but the magnitude of the differences was b2-fold lower. Motor activity was consistently
decreased in adults for all chemicals tested; however, there was more variability with the pups and clear agerelated
differences were only observed for mevinphos. These data show that three of these four OPs were only
moderately more toxic in young rats, and further support findings that age-related differences in pesticide
toxicity are chemical-specific.