Abstract: A simplex radio transmitting and receiving apparatus, has a transmission blocking means which prevents selection of the transmitting mode of the apparatus if a signal of predetermined strength is being received by the apparatus at the time the press-to-talk key is operated. An over-ride circuit enables the blocking means to be disabled in an emergency or if another transmitter has been left operating in error. The over-ride circuit can be activated by, for example, repeated operation of the press-to-talk key, and ceases to operate when the key is released to return the apparatus to its receiving mode. Also disclosed is a transmission control means which can be used to modify existing transceivers to operate in accordance with the invention. In a further feature of the invention, means are provided to detect the higher power consumption of the transceiver in the transmit mode and thereby to give a warning of an unduly extended transmission, e.g. by faulty equipment.