Background It is believed that muscarinic M1/4 receptors are closely correlated to the dopaminergic system and are strongly involved in the pathogenesis of Parkinson's disease (PD). In addition to regulating lipid metabolism and protection from stroke, statins have been used to regulate the declined cognition. We aimed to explore the regional changes in M1/4 receptors in the 6-hydroxydopamine (6-OHDA)-lesioned rat brain. Methods PD rat model was set up by injecting 6-OHDA into the unilateral medial forebrain bundle; while simvastatin (10 mg/kg/day) or saline was orally administrated for 3 weeks, respectively. Long-term memory was measured using the Morris water maze. [3H]pirenzepine binding autoradiography was applied to investigate the alterations of M1/4 receptors in the PD rat brains. Results 6-OHDA induced long-term memory deficits along with downregulation of M1/4 receptors in the hippocampus, the medial amygdala, the posteromedial cortical and the piriform cortex; simvastatin administration significantly ameliorated the impaired memory and reversed the downregulation of M1/4 receptors in the examined brain regions. Profound positive correlations were verified between the decline in long-term memory activity and the restoration of M1/4 receptors in different brain regions after simvastatin treatment. Conclusions/significance Our results provide strong evidence that M1/4 receptor modulation after simvastatin administration did, at least partially, contribute to the improvement in the long-term memory in 6-OHDA-induced PD rats. These results provide a possible mechanism for simvastatin treatment in psycho-neurological diseases such as PD via M1/4 receptors.