The interaural intensity difference (IID) created by the imposition of a cat's body in a free sound field was measured in two age groups of kittens and one group of adult cats. For all animals significantly larger IIDs were obtained for high than for low frequency stimulation. The frequency at which peak IID occurred was higher for the younger group of kittens than for either the Old Kittens or the Adults. Above 4 kHz the Adults showed a marked variation of IID with azimuth, whilst the Young Kittens showed little variation of IID with azimuth at frequencies below 10 kHz. It was found that Adult-like IIDs are achieved before physical maturation is complete. The results are discussed with reference to the physiological development of sensory systems and the implications for a possible post-natal plasticity of the auditory system are examined.