Two series of laboratory experiments were undertaken to investigate the influences of exposure duration and frequency on the toxicity of short-term pulses of ammonia to rainbow trout (Oncorhynchus mykiss Walbaum) and brown trout (Salmo trutta L.). For the investigation of exposure duration, rainbow trout were exposed to a range of unionized ammonia concentrations in single pulses of 1, 6, or 24 h, and mortality was recorded as the endpoint. For the investigation of exposure frequency, brown trout were exposed to repeated sublethal ammonia pulses of different concentrations either once or three times weekly over a period of 53 d to simulate intermittent pollution episodes. A range of endpoints were measured, including growth, gill condition, blood indices, and organ-to-body weight ratios. The results suggest that exposure duration and frequency are both important factors influencing the severity of effect in fish exposed to short-term ammonia peaks. Growth, gill condition, organ weights, and hematocrit were all significantly affected by repeated exposures, particularly at the higher exposure frequency. The results of this study confirm that the duration and return period of transient pollution events are critical dimensions when considering the development of environmental quality standards for intermittent pollution.