Monaural responses of single units isolated in the inferior colliculus of adult gerbils that have developed postnatally with one cochlea were compared with monaural responses recorded in animals that have developed with both cochleas. One cochlea of 2-day-old gerbils was ablated, and at approximately 6 mo of age, excitatory responses to stimulation of the nonoperated ear were recorded in the ipsilateral inferior colliculus. These responses were compared quantitatively with responses evoked by ipsilateral and contralateral monaural stimulation in normal gerbils. Responses to ipsilateral stimulation in adult gerbils subjected at 2 days of age to ablation of the contralateral cochlea are significantly different from ipsilateral responses in nonoperated gerbils. In several respects they are very similar to contralateral responses in nonoperated gerbils. (Differences between monaural contralateral and ipsilateral responses in control animals are documented in the companion paper, Ref. 24.) These conclusions are based on analyses of response threshold, peak discharge rate, response pattern, and minimum response latency. The mean dynamic range of ipsilateral rate/intensity functions obtained in neonatally ablated gerbils is significantly larger than the mean ipsilateral and contralateral dynamic ranges in control animals. Analyses of threshold tuning curves indicate that the frequency tuning of units in the inferior colliculus of neonatally ablated animals does not differ significantly from the tuning of units in control animals in response to either ipsilateral or contralateral stimulation. These data reveal that in normal gerbils responses of single units in the inferior colliculus to stimulation of the ipsilateral ear result in part from interactions during postnatal development between pathways that convey information from the contralateral ear. The results are discussed in terms of the known anatomic consequences of a neonatal cochlear ablation and the competition for available synaptic space in the development of the retinotectal system.