Abstract: Frequency modulation of a narrow band noise evokes slow cortical response (SCR) in guinea pigs just as pure tone modulation does. Pitch difference limen (Δf) of the narrow band noise can thus be easily obtained by measuring the SCR threshold in terms of modulation depth. In guinea pigs Δf values are found to be dependent on noise-band width (B) and central frequency (f) and the Δf-B correlation can be expressed as Δf=ABⁿ where A is the key value of Δf for a given f at B = 1Hz and, for f's of 1kHz and 4kHz, n approximates to a simple fraction of 1/3. The Δf/f ratio is essentially a constant for all examined f's ranging from 125 Hz to 8 kHz, provided B is set according to a fixed B/f rate. This is a very good demonstration of the Weber's law in respect to pitch discrimination. For expressing the overall Δf-f-B relationship in guinea pigs, a simple empirical formula, Δf=0.1f^2/3B^1/3, is proposed with which the calculated Δf values are quite close to the experimentally obtained date.