The toxicological and environmental significance of N-containing effluents discharged to seawater from fish farms is difficult to establish. Environmental quality standards for N compounds in seawater are hard to derive in the context of aquaculture because the toxicity of NH3 and NO2- to marine fish is poorly understood. Furthermore, details of aquacultural effluents are not routinely reported. Marine teleosts excrete N via the gills, skin and faeces, but do not have the metabolic capacity to cause breaches in discharge consent conditions. The most likely cause of discharge consent breaches will be poor farming practice. Nitrogen pollution will arise from food wastage, poor N absorption, and N retention. It is estimated that 52-95% of any N added to the culture system as food will ultimately pollute the environment. © 1993 Chapman & Hall.