An autoradiographic study was carried out to study the opioid binding sites in a mu-opioid receptor knock-out mouse line whose exon 2 and 3 were deleted. Mu-opioid binding sites were undetectable in this knock-out mouse line while the binding of the other two types of receptors were unaltered. Our results suggest destroying functional mu-opioid receptor does not affect the expression of the other two opioid receptors.