Periodical observations were made in 1958 in the hydrographical system of Cananéia lagoon State of São Paulo. The present paper reports the general characteristics of the regional distribution of chemical constituents in the water of the lagoon as observed on the February 11, 1958. The outer sea water (34 ‰ salinity, 4.5 ml/l O2, pH 8.4, 0.1 µg-atoms-nitrate-N/l, 0.3 µg-atoms-phosphate-P/l and 4 µg-atoms-silicate-Si/l) flows into the lagoon at flood tide through the Barra de Cananéia. The major portion of the inflowing sea water moves westward through the Baía de Trapandé pushing the inner lagoon water toward the northern sheltered region, while a smaller portion enters along the eastern waterway called Mar de Cananéia, but is apparently blocked in its flow because of the narrow passage. In the sheltered inner region the rise of the water level apparently depends upon an accumulation on the water of the tributaries which is blocked by the outer tidal wave. The lowering of the water level of the open sea during the ebb sucks out the lagoon water, and a high velocity of the outflow through the narrow waterway of the Mar de Cananéia is then usually observed. In the inner region of the lagoon when the horizontal movement of the water due to the tides is weakened, a reduction state is likely to develop in the bottom layer on account of active decomposition by bacteria of the deposited organic materials. Accordingly, considerable lowering of the oxygen tension is observed in the water overlying the bottom. In the surface water at the same station the amount of dissolved oxygen is near saturation point. The decomposing activity of the bacteria is very important for the lagoon's metabolism. The return of nutrients into the water undoubtedly contributes efficiently to enrich the phytoplankton crop.