All the existing molecular maps of M82 made up to now have revealed a double-lobed stucture interpreted as a uniform torus (Nakai et al. 1987). We have observed the central part of M 82 in the HCN (J=1−0) line with the Plateau de Bure IRAM interferometer (spatial resolution = 2.5″). The HCN molecule, due to its high dipole moment, traces clumps with a density of about 105 cm−3. For technical reasons we have divided our study in two fields. We present here the results obtained in the south-west field, covering one lobe of the “molecular ring” and the center of the galaxy in the velocity range 0-260 km s−1.