Low doses of the acetylcholinesterase inhibitors, mevinphos and physostigmine, reduce hippocampal inhibitory phenomena, measured in terms of the duration of the poststimulus inhibitory pause seen in individual hippocampal neurones in the pigeon. The cholinergic antagonist, atropine, increased hippocampal inhibition. At near threshold doses, mevinphos could, in part, reverse atropine effects. At higher doses, mevinphos synergized with atropine. In the discussion, it is suggested that any organophosphate or carbamate pesticide and its antidote, atropine, may alter "attention" or "short-term memory" through effects on hippocampus at doses too low to induce grossly detectable peripheral symptomatology. Extreme caution in the use or handling of these compounds by aerial applicators is urged. Since the effects of atropine and mevinphos can synergize, the use of atropine to permit poisoned applicators to resume activity is questioned.