The aim of this study was to explore the role of different contractile mechanisms of arterial smooth muscle, with special emphasis on those regulating Ca²⁺-sensitivity, during postnatal development. Rat saphenous arteries from newborn (NB: 5–10 day old) and adult (AD: 2.5–3.5 month old) animals were studied using wire-myography and FURA-2 fluorimetry. In vessels from AD, methoxamine (MX, 10 mM) and KCl (42 mM, in the presence of 5 mM guanethidine to block transmitter release from periarterial nerves) increase tension and [Ca²⁺] _i; the increase in tension was proportionally larger than the increase in [Ca²⁺]_i reflecting an augmentation of Ca²⁺-sensitivity. The Rho-kinase-inhibitor Y27632 at 3 mM reduced the MX- and KCl-induced increase in tension and [Ca²⁺]_i substantially; the effect on tension was proportionally larger than the effect on [Ca²⁺]_i indicating an additional, albeit not very prominent effect on Ca²⁺-sensitivity. In vessels from NB, MX (10 mM) and KCl (42 mM) increase tension but do not change [Ca²⁺]_i reflecting a considerable augmentation of Ca²⁺- sensitivity. The Rho-kinase-inhibitor Y27632 reduced the MX- and KCl-induced increase in tension without affecting [Ca²⁺] _i indicating an effect on Ca²⁺-sensitivity. Thus, the results of this study show that MX- and KCl-induced contractions of saphenous arteries are mediated by partly Rho-kinase dependent changes of [Ca²⁺]_i and Ca²⁺-sensitivity in adult animals and by almost completely Rho-kinase dependent changes of Ca-sensitivity in newborn animals. This work was supported by RFBR (grant 07–04-01527) and by the program of cooperation between Uni-Rostock and MSU.