A detailed seismic refraction survey was carried out over the Endeavour Segment of the Juan de Fuca Ridge, a medium-rate spreading center which lies off western North America, to investigate the creation and evolution of oceanic crust. A bathymetric high and the presence of hydrothermal vents suggested that the study area was the most recent locus of spreading. Travel time and amplitude data from 15 in-line air gun/ocean bottom seismometer profiles were forward modeled using asymptotic ray theory to obtain two-dimensional velocity models consisting of four primary layers which correlate well with classic models of oceanic crust. Significant lateral variations in thickness and velocities on the scale of a few to 10 km are superimposed on this basic velocity structure, but they appear to be random rather than distributed symmetrically about the ridge. We attribute them to fracturing which causes porosity changes, hydrothermal circulation which fills voids and fractures with alteration products, and variations in magmatic and/or deformational processes at the spreading center. Layer 2A is found to have low (2.6--2.8 km/s) velocities, to average 0.4 km in thickness with variations up to 0.2 km, and to be bounded at its base by a sharp velocity increase to 4.8 km/s. Along the axial ridge, velocities 0.4--0.6 km/s higher than average are interpreted for layers 2B and 2C, but these values are confined to a 2-km-wide zone centered below the ridge. Velocities along ridge-parallel lines offset 10 km are normal, indicating that maturation to off-ridge structure has occurred within at most 0.3 Ma. Layer 3 velocities decrease by 0.1--0.2 km/s for arrivals traveling along and under the axial ridge, perhaps caused by higher temperatures. However, we find no anomalously low velocities beneath the ridge, indicating that no large crustal magma chamber exists. On the basis of this study, we concluded that magmatic accretion is a fully three-dimensional process within ridge segment such as Endeavour Ridge. ¿ American Geophysical Union 1993