The behavioral toxicity of pralidoxime methanesulfonate (P2S) was examined in the rat by comparing standard measures such as conditioned taste aversion (CTA), drinking behavior and acute oral toxicity. P2S produced a weak CTA at doses of 0.4 and 0.8 g/kg (PO) and a profound CTA at the highest dose (1.6 g/kg) using a single sucrose-flavored conditioning trial with a one bottle test. The CTA produced by the highest dose of P2S was blocked by a specific, and exclusively peripheral, histamine-H2blocker, cimetidine (30 mg/kg, IP), which also has a cytoprotecting effect on gastric mucosal lesions. These data suggest that the H2 receptors may be involved in inducing the aversive effects of P2S through the inherent local irritating property of P2S on the rat gastric mucosa. There was no disruption of water drinking in thirsty rats with P2S at doses ranging from 0.2 to 1.6 g/kg. The survival time after an acute oral lethal dose of P2S (8-15 g/kg) was prolonged in non-fasted rats (16.5-38.5 min) compared to fasted ones (3.5-14.5 min), however the LD50's were identical (8.7 +/- 1.0 and 7.5 +/- 0.5 g/kg; respectively); indicating that P2S taken with food delays the lethal effects, but does not affect its lethal potency.