Recently, we demonstrated that elevated blood pressure activates mitogen-activated protein (MAP) kinases in rat aorta. Here we provide evidence that the vascular response to acute hypertension also includes induction of MAP kinase phosphatase-1 (MKP-1), which has been shown to function in the dephosphorylation and inactivation of MAP kinases. Restraint or immobilization stress, which leads to a rapid rise in blood pressure, resulted in a rapid and transient induction of MKP-1 mRNA followed by elevated MKP-1 protein expression in rat aorta. That the induction of MKP-1 by restraint was due to the rise in blood pressure was supported by the finding that several different hypertensive agents (phenylephrine, vasopressin, and angiotensin II) were likewise capable of eliciting the response, and sodium nitroprusside, a nonspecific vasodilator agent that prevented the acute rise in blood pressure in response to the hypertensive agents, abrogated MKP-1 mRNA induction. The in vivo effects could not be mimicked by treatment of cultured aortic smooth muscle cells with similar doses of the hypertensive agents. These findings support a role for MKP-1 in the in vivo regulation of MAP kinase activity during hemodynamic stress. (Hypertension. 1997;30[part 1]:106-111.)