Two-pore domain (K_2P) K⁺ channels may have a role in regulating smooth muscle contraction. For example, TASK-2 channels contribute to setting the membrane potential in smooth muscle cells from murine gastrointestinal tract (Cho et al. 2005) and rat pulmonary artery (Gönczi et al. 2006). We have investigated the expression of TASK-2 channels by RT-PCR and immunocytochemistry in rat femoral and mesenteric artery. The channel was cloned from rat kidney cDNA and expressed in Xenopus oocytes, to further define the properties of this conductance. RT-PCR analysis using TASK-2 specific mouse primers showed the presence of TASK-2 transcripts in both rat femoral and mesenteric artery whole tissue mRNA. To investigate expression at the protein level we used immunocytochemistry. Single arterial smooth muscle cells were isolated and incubated overnight with primary antibody (rabbit anti-human, 1:100 dilution, Alomone Laboratories). A goat anti-rabbit secondary antibody coupled to Alexa Fluor-488 was used for fluorescent detection in a confocal microscope. Some signal was detected in smooth muscle cells, although this did not appear to be membrane localised. Controls were by omission of primary antibody and by pre-incubation of primary antibody with antigenic peptide. To further investigate expression pattern in the artery, the antibody was tested on 8 µm frozen sections. Once again, a moderate signal was seen in the smooth muscle layer. Some evidence was found for endothelial staining, although definitive evidence awaits co-staining with an endothelial-specific antibody. In order to further characterise the TASK-2 K⁺ conductance, the subunit was cloned from rat kidney cDNA using primers based on the published mouse sequence (submitted to the EMBL Nucleotide Sequence Database, Accession number AM229406). Rat TASK-2 shows 88 and 96% homology to the human and mouse homologues respectively. The differences between the three homologues occur in the C-terminus. TASK-2 formed functional K⁺-selective and extracellular pH-sensitive channels when expressed in Xenopus oocytes (pKa = 8.56 ± 0.03). In conclusion, TASK-2 channels were shown to be present in rat femoral and mesenteric arteries by RT-PCR and immunocytochemistry, and may be expressed in both smooth muscle and endothelial cells. The channel was cloned and expressed in Xenopus oocytes, and this will facilitate future comparison with native K⁺ conductances in vascular cells.