Endothelin (ET), derived from the endothelium of blood vessels, is a potent vasoactive peptide. Although it has been reported to be involved in cardiovascular diseases, such as hypertension, the mechanism by which ET evokes vasoconstriction is still unclear. On the other hand, p42/p44 mitogen-activated protein kinase (MAPK) and p38 MAPK are activated by a variety of growth factors and cellular stresses, respectively. However, the role of p42/p44 MAPK and p38 MAPK on the ET-1-induced vasoconstriction is not fully understood. This study was undertaken to determine whether p42/p44 MAPK and p38 MAPK participate in the regulation of vascular smooth muscle contraction by ET-1. The isometric vasoconstriction and intracellular Ca(2+) ([Ca(2+)](i)) were simultaneously measured using CAF-100. Phosphorylation of myosin light chain (MLC) and p42/p44 MAPK, p38 MAPK were determined by Western blots. In rat thoracic aorta, ET-1 induced a sustained contraction. In contrast, [Ca(2+)](i) was decreased with time. Both PD98059, an inhibitor of p42/p44 MAPK, and SB203580, an inhibitor of p38 MAPK, partially attenuated ET-1-induced contractions in concentration-dependent manners. ET-1 increased phosphorylation of both p42/p44 MAPK and p38 MAPK, and PD98059 and SB203580 completely decreased phosphorylation of p42/p44 MAPK and p38 MAPK in response to ET-1 stimulation, respectively. On the other hand, PD98059 and SB203580 did not affect MLC phosphorylation in response to ET-1 stimulation. These results indicate that p38 MAPK, as well as p42/p44 MAPK, may partially regulate the ET-1-induced contraction through a MLC phosphorylation-independent pathway.