Abstract: A medical instrument (10) is used by a surgeon (S) operating on a patient (P). A surgical implement (12) is held by the surgeon performing the operation and is manipulable by him to bring a portion (14) of the implement into contact with the patient's body (B). A source (16) of ultrasonic pulses (18) produces a stream of such pulses which are supplied to the instrument. The method of the present invention enables pulses to be supplied to the instrument for a defined interval (T.sub.D) after which there is a defined interval (T.sub.E) in which no pulses are delivered. At the end of this pause pulses are again delivered to the instrument. Provision of a pause enable the surgeon to better control the operation, for example, by preventing overcutting of tissue.