The central-south area of Chile (VIII region) presents a system of bays (similar to 36 degrees 30'-37 degreesS), including the bays of Concepcion (BC), San Vicente (BSV) and Coronel in the eastern boundary of the Gulf of Arauco (SOGA). This system has been recognized as an important center for the spawning and recruitment of fish species during the winter. In the present study the hydrographic variability (temperature, salinity and density), both in space (inside and between the bays) and time (between sampling years), during the winter period (July-August) between 1993 and 1996, was analyzed. As well, its relationship to the meteorological conditions (wind and rainfall) and fluvial contribution (river discharge) was explored. At the surface level (1 m and 1-10 m depth stratum), the three bays showed spatial heterogeneity in temperature and salinity distributions, especially between the inner and the outer zones. Also, at this level, the lowest values of temperature and salinity were found in SOGA, together with a higher degree of stratification due to the presence of waters of higher dilution (> 33 psu) whereas in BC and BSV waters of lower salinity (< 33.8 psu) were predominant. Most of the hydrographic variability was attributed to the temporal dimension (P < 0.001) more than to the spatial dimension. This variability was associated with fluctuations in the wind and rainfall conditions in the area of study and in the discharge of the Biobio river in the days previous to and/or during the sampling date.