Daily subcutaneous (s.c) injections of the organophosphate diisopropylfluorophosphate caused prolonged inhibition of cholinesterase (ChE) activity in whole blood and brain and downregulation of muscarinic receptors in the central nervous system; these changes were accompanied by progressive, persistent deterioration of working memory and motor function. Further, a single s.c. injection of the organophosphate insecticide chlorpyrifos (O,O',-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothionate, CPF), caused neurochemical changes of the same magnitude and duration, but transient impairment of working memory and motor slowing. In the present study, weekly injections of CPF (0, 15, 30 or 60 mg/kg s.c.) inhibited ChE activity in whole blood of rats by 60% to 90% after 5 weeks; the highest dose also induced tremor, working memory impairment and motor slowing in daily delayed matching-to-position/visual discrimination tests. Reducing the CPF injection frequency to every other week relieved the inhibition of whole blood ChE activity (to 50%-75% of control) and ameliorated all the behavioral deficits. These studies indicate that inhibition of ChE activity by repeated injection of CPF produces a constellation of behavioral effects not evident after a single CPF treatment, even though both treatment regimens caused prolonged inhibition of ChE activity and downregulation of central muscarinic receptors.