The temporal and spatial variation of alteration in basement rocks drilled on the Mid-Atlantic Ridge (MAR) during Deep Sea Drilling Project (DSDP) Legs 37, 49, and 82 was examined using petrographic analyses of standard thin sections, X-ray diffraction (XRD) of bulk rock and clay extract powders, and selected electron microprobe analyses. Alteration at all three sites never exceeded the brownstone facies. At all sites an initial period of smectite formation was followed by a fracture-filling episode which deposited further smectite, as well as phillipsite and calcite. Alteration intensity and type do not seem to vary with depth in any of the three holes. Alteration does not increase in intensity with age, but the rocks from Holes 334 and 558 experienced a fracture filling episode not seen in Hole 411.