The objective of this work was to identify the oscillation of soybean seed moisture content, influenced by relative humidity, at Capão do Leão, Brazil, at the crop year 1991/92. During 21 days, at 8:00 and 17:00 hours, seeds samples were collected begining few days after reaching physiological maturity. Immediately after harvesting the pods were manually treshed and their seeds used for moisture determination. The dew had great influence on the seed moisture content, being in the morning always higher than in the afternoon. Both moisture levels decreased tilI the 10th day, with minimum of 16.6% and 13.6% in the morning and afternoon, respectively. After this period until to the 15th day, the moisture increased to 23.2% in the morning and 20.1% in the afternoon, exclusively in function of the high relative humidity. Afterwards the seed moisture content decreased. The seed moisture content varied among days and during the days. This variation in moisture content reached values higher than 5% during periods of no rain and at the region of Pelotas conditions. For a safe soybean seed storage, drying conditions should be avaiable due to the possibility of not reaching 13% moisture content in these experimental conditions.