Crotoxin is the main toxic component of the venom of South American rattlesnake Crotalus durissus terrificus. It consists of two components: an inactive peptide named crotapotin and phospholipase A 2 (PLA 2 ). We have previously shown that crotoxin provokes in the guinea pig isolated heart a coronary vasodilation due to PLA 2 - dependent arachidonic acid (AA) activation. In order to investigate the effect of crotoxin on Sprague-Dawley (SD) rat aortic rings, with and without endothelium, we have compared crotoxin and AA activities in the presence and absence of indomethacin or L-NNA. Crotoxin and AA both induced contractions of aortic rings, either with or without endothelium. The endothelium-independent contraction to crotoxin was more marked than one to AA. Indomethacin blocked both crotoxin and AA contractions in the presence or absence of endothelium. L-NNA was ineffective on crotoxin activity. Our data, in agreement with Mebazaa et al. (Am. J. Physiol.: 268, H250-259, 1995), demonstrate that crotoxin is able to induce a contractile response in SD rat aortic rings probably due to AA mobilization by cyclooxygenase activation. Endothelium integrity seems to have a protective effect on the underlying smooth muscle cells exposed to crotoxin.