Tropical peatlands store soil carbon constituting up to 15-19% of global peat carbon. That huge carbon pool is presently being disturbed on a large scale by land development and management, and has consequently become vulnerable. Peat degradation occurs most rapidly and massively in Indonesia's peatlands because of fires, drainage and deforestation of swamp forests. Peat burning releases carbon dioxide (CO₂) intensively but occasionally, whereas drainage increases CO₂ emissions steadily through accelerated aerobic peat decomposition. Under such circumstances, tropical peatlands might become a huge source of carbon emissions to the atmosphere. Nevertheless, the effects of drainage on the carbon balance of tropical peatland ecosystems are not well understood; more field data must be accumulated. Therefore, we measured soil respiration (RS), which is a major source of CO₂ efflux, continuously for more than one year using automated chamber systems, with consideration of microtopography, at two sites in Central Kalimantan, Indonesia: undrained and drained peat swamp forests. The RS was determined mainly by local hydrology. In the undrained forest, RS decreased sharply under flooded conditions because of anoxia. In contrast, in the drained forest, with its lower groundwater level (GWL), RS showed a quadratic relationship with GWL and gradually increased as GWL decreased when GWL was lower than about -0.8 m, which was caused chiefly by the enhancement of peat decomposition. These relationships indicate that lowering GWL by drainage increased RS, whereas annual RS was larger in the undrained forest (1347 gC m^-2 y^-1) than in the drained forest (1225 gC m^-2 y^-1) in 2005. The difference in annual RS was probably attributable to higher forest productivity in the undrained forest.