Stream water chemistry was investigated for 25 forested watersheds located about 100 km northwest of Tokyo, Japan. Stream Ca2+ and NO3- concentrations increased and Na+, HCO3-, and SiO2 concentrations decreased with increases in stream discharge rates due to the increased importance of soil water to stream water and decreased importance of groundwater during periods of high precipitation inputs. Stream chemistry was significantly different from soil water chemistry, especially during low flow, suggesting a large contribution of groundwater to the streams during these periods. There were large differences of stream chemistry among watersheds in NO3- and HCO3- concentrations (44¿30 and 114¿82 &mgr;eq L-1, respectively; average¿standard deviation), and smaller differences in Cl- concentrations (46¿8 &mgr;eq L-1). Patterns of stream NO3- concentrations were affected by the stand age, which affected the ability of the forest to retain N, and the nitrification potential, which regulated N mobility. Patterns of stream HCO3- concentrations varied concomitantly with Ca2+, Na+, and SiO2 concentrations and pH, suggesting the importance of chemical weathering of bedrock. The high concentrations of NO3- and SiO2 (191¿70 &mgr;mol L-1) and high pH (6.6¿0.5) were important attributes of the stream waters of these watersheds, due to the warm humid climate that enhanced N mineralization, nitrification, and mineral weathering. ¿ 1998 American Geophysical Union