The voltage gated L-type calcium channels (LTCCs) account for the main Ca2+ influx in vascular smooth muscle cells (VSMCs) and are critical for regulation of blood pressure. LTCC blockers are therefore commonly used in treatment of hypertension to reduce vascular tone. Long-term treatment with LTCC blockers results in structural changes of the vasculature (increased lumen diameter and decreased media thickness) thereby reversing remodeling associated with hypertension. LTCCs are also thought to affect the expression profile of VSMCs through excitation-transcription coupling. The mechanism for this is not well understood but is may involve regulation of Ca2+-activated transcription factors. In order to investigate the role of the LTCC in vascular structure and function, we have developed a method for down-regulation of the LTCC in small mesenteric arteries in vivo by the use of siRNA. Our results show a consistent decrease by ~75 % in LTCC expression which correlates with the decrease in maximum [Ca2+]i increase (75.5 %) and wall stress (85.8 % ) in response to K+-induced depolarization. The LTCC down-regulated arteries have an increased media cross-sectional area (22.4 mm ± 1.3 mm and 41.7 mm ± 4.1 mm, N = 13, control and down-regulated, respectively) in contrast to the effect of treatment with the LTCC blockers. The remodeling is associated with phenotypic changes of the VSMCs. Thus the contractile phenotype markers myocardin, tropomyosin, smooth muscle myosin heavy chain, smooth muscle a-actin and h-caldesmon are significantly down-regulated. In accordance with the VSMC phenotype change, our preliminary results with a-toxin permeabilised arteries down-regulated for LTCCs show a significantly decreased ability to respond to Ca2+. We conclude that LTCC expression and/or activity is crucial in determining VSMC phenotype as well as the structure of small resistance arteries.