Riverine inputs of fresh water, total nitrogen (TN) and total phosphorus (TP) into the Seto Inland Sea were estimated using reported values of flow rate and N, P concentrations at the lower points of rivers during 1990 to 1992. TN and TP loadings along with the fresh water discharge into all regions of the inland sea except Osaka Bay showed the increasing trend in rainy season, whereas these parameters showed no marked seasonal variation in Osaka Bay. Dissolved inorganic nitrogen (DIN) to total nitrogen (TN) ratio of river water ranged 58-76%, but the regional and seasonal variation of the ratio were not clear in the all regions. Average TN/TP ratio of river water was lowest being 24 in Osaka Bay and highest being 92 in Hiuchi Nada. The TN/TP ratio tended to be low in summer and high in winter in most regions except Hishoshima Bay. Total loadings of fresh water, TN and TP into the entire Seto Inland Sea were estimated to be 3.8 x 10^10 m3 yr-1, 7.6 x 10^4 tonN yr-1 and 6.6 x 10^3 tonP yr-1, respectively. Although the fresh water input into the Osaka Bay was 37% of total input to the entire inland sea, TN and TP loadings to the bay amounted to be 62 and 68%, respectively. This suggests that the effect of riverine input of these eutrophic substances for the coastal marine ecosystem would be severe in Osaka Bay. On the other hand, only less than 1% of total TN and TP were discharged into Hiuchi Nada, Bingo Nada and Aki Nada. On the unit area basis, fresh water input to Bisan Seto was higher than that of Osaka Bay. It is suggested that the seasonal variation of riverine input of TN and TP made clear in the present study would provide useful data to establish the model for presenting material budget among respective regions of the Seto Inland Sea and to relate habitat conditions to nutrient loading rates in future.