It has been widely accepted that upper crustal seismic velocities gradually increase from low values in the youngest crust to 'typical' crustal velocities (~5.2 km s-1) over a period of 30--40 million years, with a corresponding decrease in the observed heat flow deficiency caused by hydrothermal circulation. Some recent seismic studies, however, suggest that the seismic velocities actually rise rapidly, nearly doubling in 5 km s-1, and velocities <3 km s-1 are not observed in crust older than 5 Ma. There is no detectable systematic change in the seismic properties of the uppermost oceanic crust after 10 million years. Median and average velocities increase from 2.3 to 4.4 km s-1 by ~7.5 Ma and remain unchanged thereafter. If layer 2A is an interval capping the oceanic crust in which velocities are significantly lower than velocities that are typical of the underlying rocks (layer 2B, 5.2 km s-1), it follows that layer 2A does not disappear but is a persistent feature of the oceanic crust. These results are consistent with geochemical evidence that most hydrothermal alteration occurs within 3 million years after the formation of the oceanic crust and suggest that there is little correlation between seismic velocities and ongoing hydrothermal circulation. ¿ 1998 American Geophysical Union