Contraction of the vascular smooth muscle can be regulated by either Ca²⁺ signal or Ca²⁺-sensitivity alteration. Rho- kinase (ROCK) plays an important role in the regulation of Ca²⁺-sensitivity. ROCK inhibitors in vitro reduce vasoconstriction more considerably in newborn rats than in adults. However this problem was not investigated in vivo. Present study was thus designed to compare the effect of ROCK inhibitor fasudil (FAS) on the vasopressor response to alpha-1 adrenoceptors agonist methoxamine (MX) in newborn and adult rats. Experiments were performed on 1-, 2- and 5–6-week-old Wistar rats, anesthetized with urethane (1.2 g/kg). Catheters were implanted into the right carotid artery and right jugular vein. Influence of FAS (3 mg/kg) on the response to MX (200 mg/kg bolus injection or 400 mg/kg infusion) was studied. Similar experiments were carried out after autonomic blockade (chlorisondamine 2.5 mg/kg). FAS decreased baseline blood pressure (BP) by 10–15%, but did not change BP after autonomic blockade. FAS attenuated responses to MX in newborn rats more considerably than in adults. FAS did not alter the maximum of MX response but reduced its duration. Under autonomic blockade FAS reduced half-decay time 3-fold in adults and almost 30-fold in newborn rats. During MX infusion integrated response for 6 min from the start of infusion decreased by 65% in 1-week- old, by 27% in 2-week-old rats and by only 13% in adults. FAS also markedly blocked vasoconstriction in newborns in the experiments on the isolated saphenous artery. Thus we for the first time observed higher effect of ROCK inhibitor in vivo on the pressor response to MX in rats during early postnatal development. This work was supported by RFFI (grant 07-04-01527).