A dental unit having pressure-medium operated handpieces, a holder for each handpiece, a control unit operable to control the supply of pressure medium to each handpiece, and switch means associated with each holder and with said control unit, each switch means being operable upon removal of the respective handpiece from its holder to initiate operation of the control unit for the supply of pressure medium to the handpiece.The control unit comprises a control block corresponding to each handpiece, a first passage extending through the control blocks to supply pressure medium for operating the handpieces, a second passage in each control block communicating with a respective handpiece, a diaphragm controlling the communication between the first passage and each second passage, a control chamber controlling the operation of the diaphragm, the control chamber being charged with control pressure medium in order to move the diaphragm to a position blocking communication between the first passage and the respective second passage, a venting arrangement for venting each control chamber, and an electrically operated closure element cooperating with each venting arrangement, the closure element being controlled by the respective switch means in order to vent the control chamber when one of the handpieces is removed from its holder so that the handpiece is ready to be supplied with pressure medium via the first passage and the second passage.