This study investigates the processes that influence dimethylsulfide (DMS) concentrations over the ocean using a global three-dimensional chemistry and transport model (CTM). The model is driven by assimilated meteorological data from the Goddard Earth Observing System Data Assimilation System (GEOS-1 DAS). Results from the model are compared with DMS measurements from two marine sites, a ship cruise, and an aircraft campaign. When observed seawater DMS concentrations and meteorological conditions are used, the model reproduces the observed daily and diurnal variations of DMS concentrations at a tropical Pacific station. The model also predicts the observed changes of DMS concentrations across the Atlantic, although it overestimates the DMS level by a factor of 2. The calculated vertical DMS concentrations off Tasmania are more than 4 times higher than the measured data. The model simulates day-to-day fluctuations and interannual variations observed at Amsterdam Island but underpredicts the magnitude of the variations. Sensitivities for DMS concentrations to the parameters used in DMS emission, oxidation, boundary layer mixing, and cloud convective transport are tested. The limitations of the current model in interpreting the observations are due to (1) the uncertainties in parameterization of DMS emission from the ocean, (2) the simplistic boundary layer mixing scheme, (3) the inaccurate spatial distribution and intensity of deep convective clouds in the GEOS-1 DAS, and (4) the uncertainties in DMS oxidation rates. ¿ 1998 American Geophysical Union