Circulating sub

A circulating sub apparatus comprising: a substantially tubular outer body member having a throughbore formed therein; a substantially tubular inner body member; wherein both the inner and outer bodies normally have holes formed therein; and a displacement means for producing movement of the inner body member relative to the outer body member between: an open configuration, in which the hole(s) on the outer body member are open such that fluid is passable between a throughbore and the outside of the circulating sub apparatus via the one or more holes; and, an obturated configuration, in which the hole(s) on the outer body member are obturated; wherein the inner body member comprises a seat member adapted to catch a dropped object, and wherein the seat member is adapted to permit at least a proportion of fluid to flow past the dropped object when it is seated thereon.