We investigated how antecedent moisture conditions affect the contributions of runoff water sources in a small mountainous headwater catchment in central Japan. Spatial variations of Na(+) and Cl(-) showed that streamwater was a mixture of three components: throughfall, transient groundwater, and perennial groundwater. The contributions of end-members varied by storm event. The contributions of throughfall and transient groundwater increased and the contribution of perennial groundwater decreased when total rainfall increased. The contribution of perennial groundwater decreased when the antecedent moisture condition was drier, particularly when the groundwater level was lower. The contribution of perennial groundwater significantly decreased, and that of throughfall increased, when antecedent moisture conditions were extremely dry.