One of the forms of first musical conduct is the exploration of sound sources. When young children produce sounds with any object, these sounds may surprise them and so they make the sounds again--not exactly the same, but introducing some variation. A process of repetition with slight changes is set in motion which can be analysed, as did Piaget, as a circular reaction, but which can be seen, from a musical standpoint, as the development of a "sound discovery" by repetition and variation. It is an elementary form of "musical idea" which is a central moment in the process of invention in music. Studying these first musical conducts was the aim of a three-year research project. The chief condition of observation consisted in analysing the behaviour of one child left alone in a room exploring the sound possibilities of an instrument (a zither or a pair of cymbals). Fifty-five children from the ages of 10 to 37 months were filmed twice in this situation of solitary exploration. The videos were transcribed and analysed from various points of view by a group of 15 musician-researchers. (Contains 3 tables, 3 figures, and 2 notes.)