The economics of most modern aquaculture operations require that animals be cultured at high densities. A consequence of high‐density aquaculture is the increased probability that the animals will be exposed to elevated concentrations of nitrogenous wastes, particularly ammonia and nitrite. Ammonia toxicity is dependent primarily on the concentration of ammonia and the pH of the environment High concentrations of ammonia will decrease survival, inhibit growth, and cause a variety of physiological dysfunctions. Nitrite toxicity is dependent on the concentration of nitrite and, in many species, the concentration of chloride in the water. Nitrite toxicosis is characterized by reduced survival and growth, methemoglobinemia, and other physiological dysfunctions. Both ammonia and nitrite act as stressors in that they stimulate the release of corticosteroid hormones into circulation. Elevated concentrations of circulating corticosteroids have been linked to impaired immune function and, consequently, decreased disease resistance. Aquaculturists who wish to limit losses and remain competitive must recognize the specific detrimental effects as well as the probable immunosuppressive effects of ammonia and nitrite.