The kinetics of crossbridge phosphorylation and cellular activation rates in arterial smooth muscle are unknown, and the response rates are usually limited by agonist diffusion rates. In this study, electrical field stimulation (10–20 V AC, 60 Hz) was used to activate strips of swine carotid artery smooth muscle. Pretreatment with 0.5 μM phenoxybenzamine (PBZ) for 30 minutes significantly inhibited field stimulated stress due to nor epinephrine released from adrenergic nerves. Addition of 1–10 mM tetraethyl ammonium ion (TEA) to PBZ-pretreated tissues allowed direct stimulation of muscle cells and significantly potentate the response to field stimulation. Activation in field stimulated tissues pretreated with PBZ and TEA was strongly inhibited by the Ca2+-channel antagonist verapamil (0.5 μM for 15 minutes). Activation was rapid with myosin light chain (MLC)-phosphorylation values rising to greater than 50% of total MLC, with a half-time of approximately 1 second. Stress and stiffness development was biphasic and lagged with an estimated half-time for the first phase of 3 seconds. Stress and stiffness continued to slowly rise after 5 seconds of stimulation, while MLC phosphorylation declined significantly between 5 and 10 seconds to a nearly constant value of 35–40% for up to 60 seconds. Unloaded shortening velocity was maximal (0.067 L0/sec) at the earliest time point examined (0.5 second), prior to significant stress development and at submaximal phosphoryla tion. Significant decreases in unloaded shortening velocity were observed after 10 seconds. These results show that the kinetics of phosphorylation and dephosphorylation and the rates of activation are very rapid in this arterial smooth muscle.