The ultramafic rocks recovered from Alvin and Ocean Drilling Program studies on Conical Seamount are clasts of variably alteration and metamorphosed harzburgite and dunlte, both of which exhibit little original compositional variation. Stratigraphic relationships among the rocks within the drill holes are subtle and mainly reflect variations between the degree of alteration and metamorphism of the rocks within each hole. Examinations of fluid compositions and serpentine muds in which the ultramafic clasts are entrained provide strong evidence that serpentinization reactions are occurring in situ in the seamount. The ultramafic rocks that form the clasts in the serpentine muds of Conical Seamount represent a depleted supra-subduction-zone mantle affected by very low- to medium-grade metamorphism. Metamorphism took place as a consequence of interaction with fluids generated by dehydration of the subducted slab, both in the source region and in the body of the edifice. Subtle variations in mineral content of the rocks are consistent with interpretations of periodicity of activity associated with formation of serpentine mudflows on the flanks of Conical Seamount.