We calculate the vertical mass discharge rate from Mount St. Helens for the first few minutes of the May 18, 1980 cataclysmic eruption using a new method based on seismic constraints. The observed seismic waves indicate that the seismic source is a series of single forces. We model these forces as thrusts due to a combination of the momentum flux of the erupted products and the pressure of the eruptive jet. The momentum discharge rate is converted to a mass discharge rate based on estimates of the velocity and jet pressure as constrained by a simple fluid dynamical model. Only two parameters are necessary for the calculation: Mach number and sound velocity. The calculated mass ejected in the first 100 s is 1.6¿1011-4.6¿1011 kg. Since the total blast deposit is ~3.2¿1011-4.1¿1011 kg, one possible interpretation is that the directed blast had a significant (>30%) vertical component. ¿ 1999 American Geophysical Union