In a test of the equal-energy hypothesis, four groups of four chinchillas each were exposed to noise for 4 h at 114 dB SPL, 2 h at 117 dB, 1 h at 120 dB, and 30 min at 123 dB, respectively. The noise spectrum was designed to produce the same permanent threshold shift (PTS) at 1, 2, 4, and 8 kHz. There were no significant differences among groups in the average PTS produced (approximately 40 dB), thus confirming the earlier result of Eldredge and Covell [Laryngoscope 68, 465 (1958)] concerning the applicability of the equal-energy hypothesis to single high-intensity exposures in guinea pigs. However, it is stressed that these results do not permit us to conclude that the relation also holds for intermediate noise, or that risk of damage can be cumulated from day to day and week to week. [Supported by the National Institutes of Health, U. S. Public Health Service, Department of Health, Education, and Welfare.]