Abstract: AM-evoked slow cortical responses (SCR) were recorded in awake guinea pigs and the response thresholds (ΔI) in terms of amplitude increment in dB were determined. A qua-sirectangular pulse of 200 ms duration and with a repetition rate of I/sec served as the modulator. The typical AM-evoked SCR assumes a positive-negative-positive triphasic waveform appearing 40 -200 ms after the on-set or off-set of modulation. For white noise, for repetitive clicks (1000 pps) and for pure tones (125 Hz-16 kHz) in a very wide carrier level range (30-90 dB SPL), the ΔI values are only around 0.5 dB, quite close to those for human obtained by psychophysical methods, suggesting that the SCR ΔI values can represent the intensity difference limens for the animals, A ΔI curve for guinea pig, i.e., the functional curve of ΔI versus, I, is constructed. This ΔI curve is supposed to be the first complete and convincing one for animals.