A downhole apparatus such as multi-cycle circulating subs used during downhole drilling operations includes a piston (42) slidably mounted in a body (4) between positions in which at least one aperture (40) in the body is opened and closed. Movement of the piston (42) is controlled by a pin (86) secured to one of the body and a control member, and a control groove (52) in which a portion of the pin is received formed in the other of the body and control member. An arrangement of elements (32,76) respectively connected to the control member and body is such that, as the control member moves axially, increasing lengths of the elements locate adjacent one another so as to provide resistance to relative rotation in at least one direction of the control member and body. The relative rotation is rotation which presses the control member against the control groove. The risk of damage to the control pin is thereby reduced.