In rat aorta, endothelin-1 (ET-1) induced a slowly developing and sustained contraction in Ca(2+)-containing normal Krebs, nominally Ca(2+)-free Krebs, and Ca(2+)-free Krebs containing EGTA with a decreasing level of contraction. When ET-1-precontracted tissues were washed with Ca(2+)-free Krebs +50 microM EGTA, the tissues spontaneously and slowly relaxed. Readministration of Ca2+ during the early spontaneous relaxation phase caused a rapidly developing tonic contraction. When added during the late spontaneous relaxation phase, Ca2+ evoked slowly developing or, sometimes, biphasic contractions. In the presence of sarcoplasmic reticulum Ca(2+)-pump inhibitor, cyclopiazonic acid, the above biphasic contraction brought about by readministration of Ca2+ converted to a rapidly developing monophasic response. Thus, the first component of the biphasic contraction may be due to refilling of ET-1-sensitive Ca2+ store via the internal membrane Ca(2+)-pump. Furthermore, the ability of the phenylephrine-sensitive pool of internal Ca2+ to refill in the presence of 10(-8) M ET-1 suggests that the phenylephrine-sensitive pool differs from ET-1-sensitive pool and cannot be depleted by ET-1.