Intracellular membrane potential (Em) and force development were measured in rabbit basilar artery to help elucidate the mechanism of action of dopamine in this preparation. There was a strong correlation between membrane depolarization and contraction (r=0.95) between 3×10-7 M to 10-4 M dopamine. When the vascular muscle cells were depolarized by elevating [K]o there was a Em dependent decrease in force development in response to dopamine. Significant reduction of dopamine stimulated force development was observed when the vessel was depolarized by 5-6 mV by excess extracellular K+ and 90% inhibition was seen when the artery was depolarized to -20mv. When Ca++ influx was blocked, dopamine no longer induced force development. Such findings suggest that dopamine cotracts rabbit basilar artery by a mechanism involving membrane depolarization. This process may involve an influx of extracellular Ca++ through voltage sensitive channels.