Fluid venting from the toe of the accretionary prism off Oregon was measured in situ during a series of dives with DSRV Alvin in 1987 and 1988. A benthic chamber was placed over active vent sites to sequentially collect samples of venting fluids and to make direct measurements of discharge rates. Calibrated flow meter measurements and flow rates determined from dissolved methane transfer indicate that discharge from two vent sites, Alvin 1428 and Alvin 1900, ranges roughly between 100 and 500 L m-2 d-1, with the most reliable estimates falling in the range of 125--150 L m-2 d-1. These rates imply subsurface advective flow on the order of 100 m yr-1. Comparison of observed discharge rates with rates calculated for steady state expulsion supported by accretion-related compaction indicates that the observed flow is greater than predicted flow by several orders of magnitude. The disparity dictates that fluids are not derived locally, but are transported laterally within the prism, or that flow is not steady state and that individual vents are short-lived features in the ongoing accretion process. ¿ American Geophysical Union 1990