Caldesmon inhibits myosin ATPase activity; phosphorylation of caldesmon reverses the inhibition. The caldesmon kinase is believed to be mitogen-activated protein (MAP) kinase. MAP kinases are activated during vascular stimulation, but a cause-and-effect relationship between kinase activity and contraction has not been established. We examined the role of MAP kinase in contraction using PD-098059, an inhibitor of MAP kinase kinase (MEK). MAP kinase activity was assessed using an anti-active MAP kinase antibody and direct measurement of MAP kinase catalyzed phosphorylation of myelin basic protein, MBP-(9598). MAP kinase phosphorylation, stimulated by histamine (50 µM) or phorbol 12,13-dibutyrate (PDBu, 0.1 µM), was inhibited by PD-098059 (100 µM). PD-098059 did not alter the sensitivity or the maximal level of force in smooth muscle stimulated by histamine or PDBu, nor did PD-098059 affect contraction of -escin-permeabilized tissue. Our data suggest that p44 and p42 MAP kinases are not involved in regulation of vascular smooth muscle contraction. These results do not, however, preclude a role for other isoforms of the MAP kinase family.