Hexachlorocyclohexanes (HCHs) were determined simultaneously in air and seawater during two cruises across the Atlantic Ocean between the Arctic Ocean (NyA°lesund/ Svalbard, 79°N; 12°E) and the Antarctic Continent (Neumayer Station/ Ekstroem Ice Shelf, 70°S; 8.2°W) in 1999/ 2000. The concentrations of R-HCH and γ-HCH in air and surface waters of the Arctic exceeded those in Antarctica by 1-2 orders of magnitude. The gaseous concentrations of γ-HCH were highest above the North Sea and between 20°N and 30°S. Fugacity fractions were used to estimate the direction of the air-sea gas exchange. These showed for R-HCH that the measured concentrations in both phases were close to equilibrium in the North Atlantic (78°N-40°N), slightly undersaturated between 30°N and 10°S and again close to equilibrium between 20°S and 50°S. γ-HCH has reached phase equilibrium in the North Atlantic as R-HCH, but the surface waters of the tropical and southern Atlantic were strongly undersaturated with γ-HCH, especially between 30°N and 20°S. These findings are significantly different from two earlier estimates around 1990 as a result of global emission changes within the past decade. Therefore, we investigated the time trend of the HCHs in the surface waters of the Atlantic between 50°N and 60°S on the basis of archived samples taken in 1987-1997 and those from 1999. A decrease of R-HCH by a factor of approximately 4 is observed at all sampling locations. No decrease of γ-HCH occurred between 30°N and 30°S, but there was a decrease in the North Atlantic, North Sea, and in the South Atlantic south of 40°S. The constant level of γ-HCH in the tropical Atlantic confirms the conclusion that the tropical Atlantic acts as a sink for γ-HCH at present time. The measured R-HCH seawater concentrations were compared with results from a global multimedia fate and transport model. Whereas the time trend over 13 years and the latitudinal gradient were well reproduced by the model, the absolute levels were too high by a factor of 4.5. This may be explained by the zonal averaging employed in the model as well as uncertain emissions and degradation rates.