Previous research has suggested that acetylcholine might activate play fighting in juvenile rats through its actions on central muscarinic receptors. To test this hypothesis we evaluated the effects on play fighting by the muscarinic agonists pilocarpine and arecoline given alone or in combination with the muscarinic antagonists scopolamine or methylscopolamine. Scopolamine, but not methylscopolamine which penetrates the brain poorly, suppressed play as indexed by frequency of pinning. Pilocarpine and arecoline also suppressed pinning at higher doses. Concurrent treatment with various agonist-antagonist dose combinations produced additive rather than counteractive effects. These data do not support the supposition that central muscarinic circuits are involved in the activation of play fighting.