These studies were conducted in order to establish the dose dependency and relative peripheral versus central activity of four prototypical cholinergic antagonists on the rodent passive avoidance response, a widely used animal model of retention. Subcutaneous administration of 0.1 to 100mg/kg revealed a potency profile of scopolamine > atropine >> methylscopolarnine ≥ methylatropine for the impairment of passive avoidance responding. A series of neurological assessments of the doses used indicated that side effects alone were not sufficient to impair passive avoidance responding. Although inactive when delivered peripherally, methylatropine was able to produce retention deficits at 10 nmol (3.66 μg) when administered intracerebrally. To further evaluate whether systemic methylatropine could enter the central nervous system, either scopolamine or atropine was administered subcutaneously in mice and rats pretreated with 10–100 mg/kg methylatropine. The deficit-producing effects of scopolamine and atropine were abolished with methylatropine. Thus methylatropine is an exclusive peripheral antagonist; its ability to block the deficit-producing effects of scopolamine and atropine may occur through a change in blood brain barrier permeability or through uncharacterized pharmacokinetic properties.