Chloride (Cl) ion channels play a critical role in the response of both vascular smooth muscle (VSM) and endothelial (ENDO) cells to agonist stimulation. In VSM, agonist-induced Cl currents produce membrane depolarization, resulting in calcium influx through voltage-sensitive channels. ENDO cells also activate Cl currents after either agonist application or perturbation of cell volume. Although some of these currents have been characterized biophysically, the genes involved have not been identified. The CLCN family of voltage-dependent Cl channel genes comprises nine members (CLCN1-7, Ka and Kb) which demonstrate quite diverse functional characteristics while sharing significant sequence homology. We used Northern-blot analysis to study the expression of these Cl channel genes in cultured human aortic and coronary VSM cells and in aortic ENDO cells. CLCN3 is by far the most abundant CLC channel mRNA in both VSM and ENDO cells. Lower levels of expression are seen for CLCN2, CLCN4, CLCN5 and CLCN6. Expression levels were similar in VSM and ENDO cells except for CLCN4 which was more highly expressed in ENDO cells. In situ hybridization was used to confirm the expression of CLCN3 in intact human fetal lung. CLCN3 message was seen in VSM and ENDO cells of both large and small pulmonary vessels, indicating that their detection by Northern blotting was not an artifact of cell culture. CLCN3 is also expressed in pulmonary epithelial and bronchial smooth muscle cells but not in chondrocytes or pulmonary interstitial cells. Recent studies suggest that CLCN3 may encode the swelling-induced Cl conductance. We used whole cell patch clamp recording to demonstrate swelling-induced Cl currents in these cultured VSM cells. This suggests that the CLCN3 protein is expressed; however, the functional role of this current in VSM remains to be determined.