Assuming that transient values of an electric field and of a concentration of particles can be much higher in small local regions of a cloud than the maximum values measured within the thundercloud, the authors attempt to explain the lightning initiation mechanism. The discharges on neighbouring particles are considered in relation to the intensity of the ambient field and the distance between particles. It is shown that the electric field threshold for the appearance of breakdown discharge between close particles has lower values at certain distances between the particles than in the case of distant, well-separated particles. The first breakdown discharge between two close, slightly conducting particles enhances the electric field in the nearest space outside the discharged particles along the direction of the ambient field, so the breakdown discharge to another nearest particle in this direction is privileged. The breakdown process in a densely populated area may proceed in chain with the increasing field at the tip of the progressing discharge. Such initiating mechanism might be expected in a limited volume with reduced electric field E/p=30 V cm-1 Tr-1 and with the concentration of about 15--500 l-1 of particles from about 0.1 to few millimeters in diameter. The suggested reasoning seems to be in a fairly good agreement with the recent observations of in-cloud discharges, the structure of the cloud related to the initiation, and development of lightning flashes, and other features. ¿ American Geophysical Union 1996