Abstract: If a high frequency tone is amplitude-modulated by another sine wave of low frequency a distinct and predominating modulator pitch can be heard which changes when the mo-昫ulator frequency is in turn modulated by a rectangular pulse. Thresholds of frequency discrimination of such amplitude-modulator (Δf_am) can be measured in guinea pigs by recording the slow cortical responses evoked by its frequency-modulation. Δf_am in relation to the carrier and, modulator frequencies, to the depth and duration of amplitude modulation, and to sound intensity are systematically studied. In guinea pigs the ability of modulator pitch discrimination seems rather good. At optimal stimulus parameters the guinea pig Δf_am amounts to 3-4 Hz, quite close to its frequency differential thresholds of pure tones. Δf_em in guinea pigs and in humans (measured by psychophysical method) are compared and mechanism of modulator pitch perception and discrimination discussed.