Small pulmonary arteries < 500 µm diameter (SPA) of the cat constrict when exposed to hypoxia, whereas larger arteries > 800 µm diameter (LPA) show little or no response. It is unknown why different contractile responses occur within the same vascular bed but activator or repressor proteins within the smooth muscle cell (SMC) can modify myosin phosphatase and myosin light chain kinase (MLCK), thereby influencing the phosphorylation state of myosin light chain (MLC) and ultimately, contraction. Telokin, a protein with a sequence identical to the COOH terminal domain of MLCK is expressed in smooth muscle where in its phosphorylated state it inhibits myosin phosphatase, binds to unphosphorylated myosin and helps maintain smooth muscle relaxation. We measured telokin mRNA and telokin protein in smooth muscle from different diameter feline pulmonary arteries and whether changes in the phosphorylation status of telokin and MLC occurred during hypoxia. In pulmonary arteries telokin expression varied inversely with artery diameter but cerebral arteries showed neither telokin protein nor telokin mRNA. Although telokin and MLC were distributed uniformly throughout the SPA muscle cell cytoplasm, they were not colocalized. During hypoxia, telokin dephosphorylated and MLC became increasingly phosphorylated in SPA SMC while in LPA SMC there was no change in either telokin or MLC phosphorylation. When LPA SMC were exposed to phenylephrine MLC phosphorylation increased with no change in telokin phosphorylation. These results suggest that in SPA, phosphorylated telokin may help maintain relaxation under unstimulated conditions while in LPA, telokin‘s function remains undetermined.