The potential roles of the regulatory proteins, actin, tropomyosin (Tm) and caldesmon (CaD), i.e., the componensts of the thin filament, in smooth muscle have been extensively studied in several types of smooth muscles. However, controversy remains on the putative physiological significance of these proteins. In this study, we intended to determine the functional roles of Tm and CaD in the regulation of smooth muscle contraction by using a reconstitution system of the thin filaments. At appropriate conditions, the thin (actin) filaments within skinned smooth muscle strips of Taenia Caeci in guinea pigs could be selectively removed by an actin-severing protein, gelsolin, without irreversible damages on the contractile apparatus, and then the thin filaments were reconstituted with purified components of thin filaments, i.e., actin, Tm and CaD. We found that the structural remodeling of actin filaments or thin filaments was functionally linked to the Ca²⁺-induced force development and reduction in muscle cross-sectional area (CSA). That is, after the reconstitution of the gelsolin-treated skinned smooth muscle strips with pure actin, the Ca²⁺-dependent force development was partially restored but the Ca²⁺-induced reduction in CSA occurred once. In contrast, the reconstitution with actin, followed by Tm and CaD, restored not only the force generation but also both its Ca²⁺-sensitivity and the reversible Ca²⁺-dependent reduction in CSA. We confirmed that both removal of the thin filaments by gelsolin treatment and reconstituition of the actin (thin) filaments with Tm and CaD caused no significant changes in the level of MRLC phosphorylation. We thus conclude that Tm and CaD are necessary for the full regulation of smooth muscle contraction in addition to the other regulatory systems including myosin-linked one.