Objectives : To evaluate the effects of progesterone/progestin and in combination with estrogen in relaxation of rat aorta. Methods : Eight weeks after bilateral oophorectomy, the descending aorta of Spague-Dawley rats (n=10) were quickly removed and placed in organ bath containing Krebs solution. Each aorta ring in 2-3 mm length was connected to an isometric force transducer (FT 03, Grass, USA) and the changes in tension were recorded with an AD converter system (MP 100, Biopac Inc, USA) in a personal computer. After precontraction of the rings with norepinephrine (1 umol/L) or KCI (40 mmol/L), estradiol, progesterone, medroxyprogesterone acetate (MPA), norethisterone acetate (NETA) (10-5-10-8 M/L in each) were added to each ring and they were incubated for 15 minutes. The relaxation was expressed as a percentage of the tonic contraction. Results : Estrogen relax the aorta in all concentrations. The degree of relaxation was dose dependent (P<0.001). All of the progesterone, MPA, NETA relax the aorta and the effects was different according to the concentration of steroids (P<0.0001). The degree of relaxation was not different between estrogen and those of progesterone, MPA, NETA except MPA 10-8 M, NETA 10-5 M. Addition of progesterone, MPA and NETA to the estrogen showed similar vascular effects compared to those of estrogen alone. Conclusion : Not only estrogen but also progesterone, MPA, and NETA acutely relax aorta. Progesterone/progestin have not been found attenuate the action of estrogen in our animal in vitro study.