The distribution of the mRNA of different C-terminal splice variants of the mu-opioid receptor in rat CNS was assessed by RT-PCR. The mRNA species for MOR1, MOR1A and MOR1B were readily detectable and distributed widely throughout the rat CNS, with levels of MOR1 and MOR1A mRNA being overall greater than for MOR1B. We did not find convincing evidence that significant levels of MOR1C, MOR1C1, MOR1C2 and MOR1D are present in rat CNS. To examine possible differences in the agonist-induced regulation of MOR1, MOR1A and MOR1B, we expressed these constructs in HEK293 cells along with G-protein-coupled inwardly rectifying K+ channel subunits and measured the rate and extent of desensitisation of (d-Ala2,N-Me-Phe4,glycinol5)-enkephalin (DAMGO)- and morphine-induced G-protein-coupled inwardly rectifying K+ currents. Morphine-induced desensitisation was rapid for all three splice variants (t1/2: 1.2-1.7 min) but DAMGO-induced desensitisation was significantly slower for MOR1B (t1/2 4.2 min). Inhibition of endocytosis by expression of a dynamin-dominant negative mutant increased the rate of DAMGO-induced desensitisation of MOR1B. These data show that some splice variants of mu-opioid receptor are widely expressed in rat CNS but question the existence of others that have been reported in the literature. In addition, whereas the rate of desensitisation of MOR1 and MOR1A is agonist-independent, that for MOR1B is agonist-dependent.