The vasoactive effecs of ketamine on aortic and pulmonary arteries have not heen clearly characterized. Nevertheless, it has been recommended to avoid ketamine in systemic and pulmonary hypertension because of its tendency to increase systemic and pulmonary vascular resistance. This study was designed to investigate and compare the direct effects of ketamine on isolated rat aortic and pulmonary arteries, with or without intact endothelium. The optimal resting tension (Lmax) of each ring was searched hased on contractile responses to 3.7X10(6)M norepinephrine. Once the Lmax was Obtained, the peak developed tension was recorded as the control. Thereafter, in the second part of the experiments, prior to ketamine exposure, the endothelium was denuded which was confirmed pharmacologically using norepinephrine(3.7X10-6M) and acetylcholine(10(-6)M). In groups with intact endothelium, .3X10(3)M ketamine relaxed aortic and pulmonary artery ring by -10.3+/-5.6%, -17.8+/-4.4%, respectively. In groups without intact endothelium, 3X10(3)M ketamine relaxed aortic and pulmonary artery ring by -9.9+/-3.6%, -14.2+/-3.8%, respectively. It was statistically significant. In groups with or without intact endothelium, 0.1X10(3) M ketamine relaxed aortic and pulmonary artery ring. Hut it was statistically insignificant. We conclude that ketamine is a powerful aortic and pulmonary artery dilator in vitro and that is endothelium independent.