Seasonal variation of ammonium flux and nitrite+ nitrate flux from the sediments at two stations located in the inner and central areas of Hiroshima Bay, Japan were studied from August, 1994 to May, 1995. The ammonium and nitrite +nitrate fluxes were calculated from the concentration gradient between the sediment porewater and the overlying water. Ammonium flux was always higher in the inner area than in the central area, showing 22. 2-60. 8μg atN m-2 h-1 in the inner area and 18. 2-41. 0 μg atN m-2 h-1 in the central area, respectively. These fluxes were the maximum in August and the minimum in January at both stations. Nitrite + nitrate flux did not show any significant difference between locations, showing 0. 47— 17. 1 μ g atN m-2 h-1 in the inner area and 0. 24 —18. 2 μg atN m-2 h-1 in the central area. These fluxes were the highest in January, and the lowest in October at both stations. Ammonium flux showed significant relationships to the environmental parameters, such as temperature, and redox potential of the sediment and dissolved oxygen concentration in the overlying water. It was assumed that the am mounium flux might increase with increasing of the ammonium concentration in the porewater accompanied by the increase in decomposition rate of organic material in the sediment. The increase of mud temperature might have accelerated the decomposition of organic material in the sediment, resulting the consumption of oxygen and reduced condition of the sediment. On the other hand, nitrite +nitrate flux showed no relationships to the environmental parameters, although it depends on the nitrite +nitrate concentration in the porewater. From these results, the mechanisms of nitrite+nitrate release was assumed to be mainly governed by the gradient between their concentration in the sediment and those in the overlying water.