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To study natural convection and deep water formation, in the open ocean, direct measurements of vertical motions and of the hydrological structure have been made during winter in the northern part of the Western Mediterranean Sea. The measurements were made in the winters of 1969, 1970 and 1972, using neutrally buoyant rotating floats described by Webb, Dorson and Voorhis (1970). Vertical profiles of the potential density and salinity in the vicinity of the floats were computed from in situ measurements of electrical conductivity, temperature and pressure. The 1972 measurements show that, in conditions characterized by a weak density stratification, there is no vertical motion at the local inertial period. The dominant carriers of energy are stability waves ( Gascard , 1972), i.e. gravity-inertial waves, probably induced by bursts of wind at the surface and generated in the sloping region of the continental shelf. Then, the 1969 and 1970 measurements ( Stommel, Voorhis and Webb , 1970, 1971) suggest that, in conditions of no density stratification, winds, the Earth's rotation and tides have a great influence on the stability waves and may result in natural convection and subsequent deep water formation. Both were observed in 1969 and 1970.