The amplitude modulation threshold for sinusoidally modulated noise was measured in chinchillas using stock avoidance conditioning procedures. Chinchillas were initially conditioned to respond to noise bursts and their thresholds for the noise bursts were measured. Then, the chinchillas were conditioned to respond to amplitude modulations in a continuous noise. The modulation frequencies were between 2 and 2048 Hz. The modulation thresholds were approximately 9%– 10% at modulation frequencies below 16 Hz. At higher modulation frequencies, the threshold increased and approached a limiting value of approximately 40%‐50% near 1 kHz. Modulation thresholds were also measured in human listeners using the same experimental apparatus. The thresholds for humans were lower than those for the chinchilla at modulation frequencies below 64 Hz. [Supported in part by NSF, NIOSH, NIEHS.]