The Plinian phase of the May 18, 1980, Mount St. Helens eruption is modeled as a steady state discharge of dacitic magma from a reservoir at 7--10 km depth at a rate of 1.94¿107 kg/s. Properties of the magma, including preeruption volatile content (4.6% in the melt), temperature (920¿--940¿ C), and confining pressure (190--250 MPa) are constrained by petrologic studies. Mass eruption rate, magma viscosity, and independent estimates of magma ascent velocity suggest a 95-m-diameter conduit at a depth below vapor saturation. Dispersal of pyroclasts indicate a minimum exit velocity of roughly 200 m/s during the Plinian phase. An upper limit of 330 m/s is obtained from the total amount of exsolved volatiles. Model-derived vent diameters based on 0.1-MPa exit pressure, petrologically inferred magma properties, and known mass eruption rate range from 105 to 135 m with a flared configuration. The calculated vent diameter, mass eruption rate, and exit velocity define conditions close to the transition between convective column rise and column collapse. These results are consistent with the sporadic generation of pyroclastic flows during a period characterized mainly by a sustained eruption column.