Tone‐on‐tone masking and differential electrode techniques have been employed to investigate the tuning properties of the DIF summating potential (SP). In these experiments the SP is elicited by tone bursts of set frequency and intensity while a second tone of variable frequency and level is used to reduce SP magnitude to a desired degree. The SP tuning curve is generated by frequency—intensity pairs formed by the masker that yield a preset decrease. Results reveal that DIF and whole nerve action potential (WNAP) tuning curves are quite similar while tuning properties of the DIF+are more analagous to CM interference functions. The dependence of various features of these SP tuning curves upon stimulus frequency and level are detailed in the present work. [Work supported by NINCDS.]