1. Previous studies of the superior olive of echolocating bats suggest that the lateral superior olive (LSO) retains the same structure and function as in other mammals but that the medial superior olive (MSO) is different in structure and possibly also in function. The present study is an examination of this idea in Pteronotus parnellii, a bat that has a large and well-defined MSO. 2. Using pure tones presented via earphones, we obtained data on frequency tuning for 60 single units and 96 multiunits in LSO and 94 single units and 154 multiunits in MSO. Of these we also obtained binaural response characteristics from 55 single units in LSO and 72 single units in MSO. 3. LSO and MSO each have a complete tonotopic representation, arranged in a sequence similar to that of other mammals studied. However, in both LSO and MSO there is an expanded representation of the frequencies around 60 kHz, the main frequency component of the bat's echolocation call; there is another expanded representation of the range around 90 kHz, the third harmonic of the call. The expansion of these frequency ranges suggests that the functions of LSO and MSO in Pteronotus are related to echolocation behavior. 4. The binaural characteristics of cells in LSO were essentially the same as those seen in other mammals. Most LSO units (93%) were excited by the ipsilateral ear and inhibited by the contralateral ear. The responses of nearly all LSO units were completely suppressed when the sound level at the two ears was equal. 5. The binaural characteristics of cells in MSO were different from those in nonecholocating mammals. Most MSO units (72%) were excited by the contralateral ear but were neither excited nor inhibited by the ipsilateral ear. Of the remaining units, 21% were excited by the contralateral ear and inhibited by the ipsilateral ear, and only 6% were excited by both ears. 6. The temporal discharge patterns of units in MSO differed from the tonic response pattern seen in LSO. Most MSO units had phasic response patterns, with a few spikes at the onset or offset of the stimulus; the response often changed from ON to OFF depending on stimulus frequency. 7. The results support the idea that in evolution LSO has remained unchanged, whereas MSO has undergone adaptation. The function of LSO in Pteronotus seems to be identical to that in other mammals, i.e., analysis of interaural sound level differences to derive azimuthal location. The function of MSO in Pteronotus must be different from that in nonecholocating mammals.(ABSTRACT TRUNCATED AT 400 WORDS)