Abstract： Objective To investigate the sensitivity of common carotid arteries of the rats to ceramide (Cer) regulation after simulated weightlessness.Methods Fourty-eight male Sprague-Dawley rats that matched for weight were randomly assigned to suspended group (n=24) and control group (n =24).Tail suspension of rats was carried out for 4 wk to simulate the effect of weightlessness.Isometric force recording system was used to detect the vasoconstriction and vasodilation response of common carotid arteries ring.Dihydroethidium fluorescent probe was used to evaluate the level of superoxide anion (O2-·) in arteries.Results Vasoconstriction response and vasodilation response showed insignificant difference between suspended group and control group.Incubation with C6-ceramide (C6-Cer) or Cer antibody (anti-Cer) did not change the vasoconstriction response and O2-· level of CA in control group but increased Cer level,vasoconstriction response to KCl and O2-· level of common carotid arteries in control group,and did not change the vasodilation response of common carotid arteries both in control and suspended group.In suspended group,C6-Cer incubation increased vasoconstriction response of common carotid arteries both to potassium chloride and phenylephrine,as well as the O2-· level (t=6.077-12.630,P＜0.05) but less to acetylcholine and nitroprusside sodium.The anti-Cer incubation decreased response vasoconstriction of common carotid arteries in suspended both to potassium chloride and phenylephrine,as well as the O2-· level (t=11.582 32.130,P＜0.05) but the vasodilation response.The incubation of superoxide dismutase or hydrogen peroxide depressed the changes of vasoconstriction response caused by C6-Cer or anti-Cer.Conclusions Cer regulation increases the sensitivity of common carotid arterial constriction in the simulated weightless rats and the mechanism is concerned with reactive oxygen level.