Abstract: There is provided a circulating sub apparatus (50) having a tubular outer body member (51, 53, 54, 55, 56, 57) and a tubular inner body member (52, 65). The outer body member (51, 53, 54, 55, 56, 57) and the inner body member (52, 65) each have one or more holes (66, 68) substantially transverse to the longitudinal axis of the outer (51, 53, 54, 55, 56, 57) and inner (52, 65) body members. A displacement mechanism produces relative movement between the outer (51, 53, 54, 55, 56, 57) and inner (52, 65) body members such that the outer (51, 53, 54, 55, 56, 57) and inner (52, 65) body members may be repeatably moved between an aligned position, in which the one or more holes (66, 68) on the inner body member (52, 65) are aligned with the one or more holes (66, 68) on the outer body member (51, 53, 54, 55, 56, 57), and an obturated position, in which the one or more holes (66, 68) on the inner body member (52, 65) are obturated by the outer body member (51, 53, 54, 55, 56, 57).