Lithium (Li) has been previously reported to increase acetylcholine turnover and release in rat brain and to potentiate the neurotoxicity of cholinergic agents. We studied the effect of chronic Li administration, alone and in combination with the muscarinic antagonist, scopolamine, on two cholinergically-mediated responses and on muscarinic cholinergic receptor (MCR) binding in rat brain. Administered separately, Li and scopolamine enhanced the cataleptic and hypothermic responses to pilocarpine; combined administration resulted in an additive effect on both these measures. [3H]Quinuclidinyl benzilate ([3H]QNB) binding was increased by Li in the corpus striatum but not in the cortex, hippocampus and hypothalamus. Scopolamine increased [3H]QNB binding in the striatum, cortex and hippocampus; Li and scopolamine effects on striatal MCR were not additive. Contrary to a previous report, antagonist-induced MCR supersensitivity was not prevented by concurrent Li administration in any of the brain areas studied. The additive effect of Li and scopolamine on pilocarpine-induced catalepsy and a trend in this direction for pilocarpine-induced hypothermia suggest that the actions of the two agents to enhance cholinergically mediated responses may be achieved by different mechanisms. Supersensitive responses following scopolamine may be attributed to antagonist-induced up-regulation of postsynaptic muscarinic receptors as demonstrated in the binding studies. The effects of Li to enhance cholinergically-mediated catalepsy and hypothermia are interpreted as extending previous reports that Li stimulates brain cholinergic function by a presynaptic increase in acetylcholine turnover and release.