CONCLUSIONS: We conclude that: (1) among several cues examined, the monaural cue of direct-to-reverberant (D/R) ratio in the ipsilateral ear provides the most information about sound-source distance; (2) interaural level difference (ILD) provides less information about sound-source distance; and (3) a comprehensive theory of three-dimensional auditory localization must incorporate the fact that all of the major acoustic cues change with distance. OBJECTIVE: Neural mechanisms underlying auditory localization of distance are poorly understood. The present study was an initial step toward filling this gap in knowledge. MATERIALS AND METHODS: The binaural room impulse responses of adult barn owls were measured. The sound source was placed at various distances (up to 80 cm) and azimuths (0-90 degrees) relative to the owl's head, with the elevation kept at 0 degrees . RESULTS: We determined the value of each cue for a 3-10 kHz band, and found that: (1) D/R ratio of signal amplitudes provided the most information about sound-source distance; (2) the ipsilateral D/R ratio represented distance more clearly than the contralateral or binaural-average D/R ratios; (3) ILD of direct signals increased with decreasing distance under certain conditions; (3) interaural time difference (ITD) of direct signals increased with decreasing distance at 90 degrees azimuth; and (4) the spectral patterns of ILD and the monaural direct signals changed with distance in complex ways.