Most toxicity tests investigate constant concentrations of a chemical. Concentrations of many compounds in the environment are dynamic, however, and individuals may be more sensitive to pulses of a chemical initiated at specific points during development. Realistic toxicity tests are important to accurately assess the toxicity of a compound. Bufo americanus and Hyla chrysoscelis tadpoles were exposed to concentrations of nitrate, a pervasive stressor in the environment, up to 5 mg/L of NO3-N in constant concentrations and in pulses at three different points during development. At the termination of the experiments, individuals were measured for developmental stability (DS) and traditional fitness correlates (size, body condition, and time to metamorphosis). No significant differences were found in these measures between treatments and controls in B. americanus tadpoles. In H. chrysoscelis, however, more extreme directional asymmetry was found in the middle and late pulses, indicating that the sudden change in concentration of nitrate decreased the level of DS in tadpoles. These results indicate that nitrate has subtle but important effects at low doses, and they suggest that species may be better able to deal with pulses that occur early rather than late in development. A greater understanding about the effects of pulses will help conservation biologists to manage populations and prevent population declines.