Fish excretions and the transformation of nitrogen by bacteria in the nitrifying biofilter are two of the main sources of dissolved inorganic nitrogen (DIN) in fish farms that use recirculating water systems. In this study, the DIN concentration in an experimental Dicentrarchus labrax aquaculture system was calculated using empirical sub-models for fish growth, ingested food and water replacement. The specific growth rate (SGR) (% day(-1)) and the daily feeding rate (DFR) (% day(-1)) both depend on the average weight, W (g), of the fish: Y = aW(b), where Y may be SGR or DFR, and a and b are empirical constants. The DIN discharge rate, Gamma(N) (% of ingested nitrogen), in the experimental aquaculture system was expressed as a function of increasing replacement water flow rate, theta (day(-1)): DIN = c theta(d), where c and d are empirical constants. Only three variables (the number of fish, the initial fish weight and the replacement water flow rate) are required to run the general model, which was tested over a period of 12 months (June 1997-June 1998). This model, calibrated and validated on independent sets of data obtained from the same experimental system, accurately predicted the concentration of DIN in the effluent (r(2) = 0.92).