The chronic effects of ammonia to rainbow trout Salmo gairdneri were studied in a laboratory test conducted over a 5-year period. Fish were tested at five concentrations over the range 0.01–0.07 mg/liter un-ionized ammonia; the mean pH of the test water was 7.7, and the mean temperature was 9.3 C. Parental fish were exposed for 11 months, the first filial generation (F1) for 4 years, and the second filial generation (F2) for 5 months. The parental fish spawned of their own volition at all ammonia concentrations tested; baskets containing crushed rock served as the spawning substrate. The F1fish did not spawn voluntarily at either 3 or 4 years of age, although manual spawning of 4-year-old F1fish produced viable eggs. There was no significant correlation between ammonia concentration and numbers of egg lots spawned, total numbers of eggs produced, numbers of viable eggs, growth of progeny, or mortality of parents or progeny in any of the generations tested. Blood ammonia concentrations were measured in F1fish, and proved to be positively correlated with ammonia concentrations in the test water. Histopathological lesions were common in parental and F1fish at un-ionized ammonia concentrations of 0.04 mg/liter and higher; in F2fish, which incurred a severe protozoan infection (Costia sp.), lesions were common at 0.02 mg/liter and higher.