The effect of chronic morphine exposure on the synaptic plasma-membrane subproteome in rats was studied by the isotope-coded affinity tag (ICAT) method coupled with capillary reversed-phase liquid chromatography/electrospray ionization mass spectrometry and tandem mass spectrometry. ICAT-labeled tryptic peptides of synaptic membrane proteins were successfully identified using tandem mass spectrometry in conjunction with protein database searching. Several important synaptic plasma-membrane proteins displayed significant regulation changes as a result of chronic morphine exposure in vivo. In particular, an integral membrane protein Na+/K+ ATPase (α-subunit) involved in regulation of the cell membrane potential by controlling sodium and potassium ion permeability was downregulated by 39 ± 2%. This result was in excellent agreement with the reduction in electrogenic Na+, K+ pumping due to about 40% downregulation of Na+/K+ ATPase α3-isoform in myenteric S-neurons of morphine-exposed guinea-pigs measured by others via immunohistochemistry. The decrease in the abundance of non-erythroid αII-spectrin in the synaptic plasma-membrane fraction was also observed, which was hypothetically associated with the breakdown of the protein due to the upregulation of the proteolytic enzyme caspase-3 upon chronic morphine exposure. Copyright © 2005 John Wiley & Sons, Ltd.