Summating potentials (SP) in response to tone bursts were measured from all three scalae of the first, second, and third turns of guinea pig cochleae. Both the intensity and frequency of the stimulus were varied over a wide range of values, and averaging techniques were used to obtain the summating potential. The scala media response (SM) was compared to the scala tympani (ST) and vestibuli (SV) responses, as well as to the potential gradient across the cochlear partition and the common potential of the two perilymphatic scalae. The former of these quantities is defined asDIF = (SV−ST)/2, while the latter asAVE = (SV+ST)/2[Dalloset el., Science170, 641–644 (1970)]. It was demonstrated that at any cochlear location the potential gradient across the cochlear partition (DIF) and the scala media response are best correlated in the dependence of response magnitude and polarity upon stimulus parameters. Aside from the similarities in the response patterns, some significant differences were also noted. Probably the most important of these is that the width of the characteristic negative summating potential bands in the vicinity of the best frequency for a given electrode location is greater in SM than when recorded from the perilymph with the differential electrodes. [Supported by grants from the National Institute of Neurological Diseases and Stroke.]