Aims: It is well known that sympathetic innervation (SI) of arterial vessels in rats develops only after birth. A plexus of fibers and varicosities appears over the surface of the vessels during the first postnatal week, and reaches the adult pattern 4-5 weeks after birth. Along with that, the mechanisms of arterial smooth muscle contraction change during postnatal development. In particular, expression of Rho-kinase (RhoK) that augments contractile machinery sensitivity to Ca²⁺ in adult rats is smaller than in newborns. The aim of this study was to investigate if the role of RhoK in the regulation of smooth muscle tone also diminishes during postnatal development and correlate this alteration with the state of SI. Methods: We used five groups of Wistar rats: 1- week-old (SI is very sparse), 2-week-old (the period of SI establishment), 4-5-week-old (SI is developed); and two groups of 7-8-week-old rats: intact and neonatally sympathectomized (SX) with guanethidine. We studied contraction of saphenous artery ring segments in response to a₁-adrenoceptor agonist methoxamine (MX, 0.1-100 microM) in control and in the presence of RhoK inhibitor fasudil (FAS, 10 microM) using wire-myography. Results: In all groups of rats FAS depressed the contractile response to MX; this was evident from rightward shift of concentration-response curve together with reducing its maximum. The effect of FAS was most prominent in 1-week-old group. In two older groups (2-week and 4-5-week-old) the effects of FAS did not differ and were not as strong as in 1-week-old group. Inhibitory effect of FAS was smallest in 7-8-week-old groups, but vessels from SX rats were more sensitive to RhoK inhibitor than intact. Conclusion: The contribution of RhoK in arterial smooth muscle contraction reduces during aging and this reduction is associated with SI development. This work was supported by RFBR (grant 07-04- 01527).