A behavioral response paradigm was used to measure masked underwater hearing thresholds in five bottlenose dolphins and two white whales before and immediately after exposure to intense 1-s tones at 0.4, 3, 10, 20, and 75 kHz. The resulting levels of fatiguing stimuli necessary to induce 6 dB or larger masked temporary threshold shifts (MTTSs) were generally between 192 and 201 dB re: 1 microPa. The exceptions occurred at 75 kHz, where one dolphin exhibited an MTTS after exposure at 182 dB re: 1 microPa and the other dolphin did not show any shift after exposure to maximum levels of 193 dB re: 1 microPa, and at 0.4 kHz, where no subjects exhibited shifts at levels up to 193 dB re: 1 microPa. The shifts occurred most often at frequencies above the fatiguing stimulus. Dolphins began to exhibit altered behavior at levels of 178-193 dB re: 1 microPa and above; white whales displayed altered behavior at 180-196 dB re: 1 microPa and above. At the conclusion of the study all thresholds were at baseline values. These data confirm that cetaceans are susceptible to temporary threshold shifts (TTS) and that small levels of TTS may be fully recovered.