Caldesmon phosphorylation has been proposed to be involved in regulation of smooth muscle contraction. Mitogen-activated protein (MAP) kinase has been suggested to be the caldesmon kinase; stimulation-induced MAP kinase activation in intact vascular smooth muscle, however, has not been demonstrated. We measured temporal profiles of MAP kinase activation in response to histamine stimulation and membrane depolarization in intact swine carotid artery. Phosphotyrosine levels of 42- and 44-kDa MAP kinases were elevated during contraction in response to histamine or KCl. The temporal profile of MAP kinase activation/inactivation was similar to that for contraction/relaxation of the vascular tissue in response to KCl or histamine stimulation. MAP kinase activated during contractile stimulation phosphorylates caldesmon with a specific activity significantly greater than that for myelin basic protein-(95-98). We propose that MAP kinase is activated in response to all forms of contractile stimulation. We also suggest that activated MAP kinase phosphorylates and disinhibits the effects of caldesmon on actin-myosin interactions. This disinhibition allows an inherent level of myosin ATPase activity to be expressed.