AbstractRicin, a toxic lectin from castor beans, reduces blood pressure. The current studies determined the effect of ricin on contractions in response to norepinephrine (NE) of rabbit central ear artery, endothelium‐dependent relaxations to methacholine and ATP of aorta rings and endothelium‐independent relaxations to papaverine in central ear artery and aorta rings. Rabbits were given 0.11 or 0.22 μg kg−1ricin i.v. and 18 h, 4 days, or 7 days later the arteries were removed and tested. Maximal contractions to NE were increased by 8–23% (P>0.05) with ricin treatment. The EC50was increased in five of six ricin‐treated groups, although only significantly so at 18 h after a minimum lethal dose of ricin. Maximum relaxations to methacholine were increased by 20–57% (P>0.05) at 18 h following both ricin doses but returned to control values at later time points. Relaxations in response to ATP were significantly enhanced in all ricin‐treated groups (95–205%) except in the 18 h and 4‐day 0.11 μg kg−1dose groups where the increase was 0–46%. Relaxations in response to papaverine were not altered in either artery. Thus, ricin decreases the sensitivity of the rabbit central ear artery to NE and increases endothelial‐dependent relaxations of the rabbit aorta. Therefore, decreased blood pressure following ricin administration may be due to vasodilation caused by decreased vascular contractions and increased endothelial‐de