Summary A detailed seismic refraction experiment has been performed on the MidAtlantic ridge near the median valley at 4YN, using two ships and anchored sono-radio buoys. Both the slope-intercept and the time-term methods were used to interpret the data. The time-term analysis indicated a continuous M discontinuity at a mean depth of 7.5 km with a mean velocity of 7.9 km s-' for the underlying material. This result was substantiated by the time-distance plots with the exception of the line of shots nearest the median valley which gave a velocity of 7.5 km s-'. Evidence for anisotropy of the 7.9 kms-' material was found, the velocity deviation being k0.25 km s-' and the azimuth of maximum velocity, 080". Two crustal layers, the upper having velocities in the range 3.5 to 5.4 km s-l and the lower velocities around 6.6 km s-' were observed but one or both of these are absent on many of the time-distance plots. No evidence was found for anomalous mantle material except within the immediate vicinity of the median valley and a density model of the upper mantle beneath the ridge which is consistent with this observation and other recent geophysical measurements is presented. The model implies an upwelling of partially melted mantle material from the low velocity zone. The composition of the oceanic crust is also discussed in relation to the seismic results and to the physical properties of the rocks dredged from the sides of seamounts in the area. The observations indicate that layer 3 cannot be composed of serpentinite but that it is probably composed of basic igneous rocks.