Previous pharmacological studies have indicated the possible existence of functional interactions between opioidergic and glutamatergic neurons in the CNS. In the present study, [(3)H]AMPA binding and the expression of mRNAs encoding flip and flop variants of three subtypes of AMPA glutamate receptor GluR1-3 were examined by in situ hybridization technique in order to investigate whether there is a change in the AMPA receptor system of mice lacking the mu-opioid receptor. In the mu-opioid receptor knockout mice, [(3)H]AMPA binding was increased in the hippocampal CA1 and dentate gyrus, cortex, and caudate putamen compared with that of the wild-type animals. The expression of GluR1 flip mRNA was increased in the cortex and caudate putamen of mu-opioid receptor knockout mice. The expression of GluR1 flop mRNA was increased in the cortex, caudate putamen, and hippocampal CA1 layer of mu-opioid receptor knockout mice. The expression of GluR2 flip mRNA was decreased in the hippocampal dentate gyrus of mu-opioid receptor knockout mice. The expression of GluR2 flop was not altered in any regions studied. The expression of GluR3 flip was increased in the cortical area and caudate putamen of mu-opioid receptor knockout mice. The expression of GluR3 flop was increased in the cortical area, hippocampal CA3 area, and caudate putamen of mu-opioid receptor knockout mice. These results indicate that [(3)H]AMPA binding and the expression of GluR1-3 mRNA were increased in a region and subunit specific manner, and suggest that changes in the AMPA receptor system are accompanied by the absence of mu-opioid receptor gene.