The development of calls (quantified by a series of acoustic parameters) of (a) normal, (b) socially isolated, and (c) deafened kittens that were released in four different situations has been studied from birth to 170 days of age. All call parameters studied except noise components show developmental changes that can be related to the development of (a) the vocal tract (fundamental frequency, harmonic with maximum intensity, upper-frequency limit and frequency range, occurrence of frequency, and intensity modulations), (b) feedback control through the auditory system (sound-pressure level, harmonic with maximum intensity, call-variability), and (c) motivational valuation of the releasing situations (duration). Isolated and deafened kittens displayed quantitative differences in certain call parameters compared with normal animals. Calls of deafened animals are, on the average, louder, more tonal and uniform, and differentially pitched compared with those of normal, hearing animals.