Potassium (K+) channels play an important role in mediating pulmonary vasodilation caused by increased oxygen tension, nitric oxide, alkalosis and shear stress. To test the hypothesis that lung K+ channel gene expression may be altered by chronic increases in pulmonary blood flow, we measured gene and protein expression of calcium-sensitive (Maxi-K_Ca) and voltage-gated (Kv 2.1) K+ channels, and a pH sensitive K+ channel (TASK), in distal lung from fetal lambs in which an aorto-pulmonary shunt was placed at 139 days gestation. Under baseline conditions, animals with an aorto-pulmonary shunt showed elevated pulmonary artery pressure and pulmonary blood flow compared to twin controls. Hypoxia caused a greater increase in pulmonary vascular tone in shunt animals, compared to controls. Alkalosis caused pulmonary vasodilation in control, but not shunt animals. To determine lung K+ channel mRNA levels, we performed quantitative RT-PCR. In comparison with control animals, lung K_Ca channel mRNA content was increased in shunt animals, while TASK mRNA levels were decreased. There was no difference in Kv 2.1 mRNA levels. Channel protein expression was consistent with these findings. We conclude that in the presence of elevated pulmonary blood flow, K_Ca channel expression is increased and TASK is decreased.