Certain differences in cochlear anatomy between the agoutiDasyprocta puctataand guinea pigCavia porcellusprovide an empirical test of models dealing with inner‐ear mechanics. The basilar membrane of agouti, for example, is about 60% longer and somewhat narrower than that of the guinea pig. Based on the work of von Békésy and others, velocity of the traveling wave should be greater in the agouti than it is in the guinea pig. In addition, a greater proportion of the basilar membrane of the agouti should be devoted to reception of higher‐frequency stimuli. The cochlea microphonic (CM) was used as the index of basilar membrane response in 18 agoutis and 12 guinea pigs. Scala media recordings were made from each turn, and differential recordings were made from each turn but the apex in both species. CM latencies to tone pips and clicks were measured. Sensitivity functions were also taken. As predicted by several models, latency per unit length of basilar membrane was shorter for agoutis than for guinea pigs. For each turn agouti sensitivity to low‐frequency tones was less and to high‐frequency tones greater than was that of the guinea pig.