GABA-like immunoreactivity was studied in surface preparations of cochleas from postnatal developing mice, and GAD-like immunoreactivity was studied in the adult. GABA-positive fibres are already present at birth; they innervate both the inner and outer hair cells and some spiral ganglion cells. The GABA-positive fibres that enter via the intraganglionic bundle send collaterals to the spiral ganglion and to the hair cell region. Fibres that enter along the central processes of spiral neurons end predominantly among the spiral ganglion cells. A few spiral neurons display pericellular rings of GABA-positive boutons at birth. In older animals, the endings occur on a small number of spiral ganglion cells either as rings or as brush formations. The early GABA-positive fibres reach the inner hair cells around the second day and the outer hair cells (of the upper turns only) around the seventh day. In 12-day animals, tunnel fibres arborize in the outer hair cell region; their collaterals make contacts with the outer hair cells within four to eight cell-wide segments, distributing the endings high, up to the reticular plate. In older animals, fibres (both GABA-and GAD-positive) may innervate single vertical rows of outer hair cells. In the maturing and the adult cochlea, the GABA-positive component of the inner spiral bundle is conspicuous and extends along the entire cochlear length, but the innervation of the outer hair cells comprises only the mid and apical turns. GABA-positive nerve cells occur among the small vestibular neurons, occasionally among the cells of eighth nerve nucleus and only exceptionally in the spiral ganglion. In the adult animal, GAD-positive cells, although uncommon, were observed among the spiral neurons. In the developing animal, GABA-positive fibres give rise to transitory formations: (1) a convoluted plexus running beneath and among the radial bundles and (2) a sparse plexus, continuous with the inner spiral bundle and running in the upper plane of the inner spiral sulcus. GABA-like immunoreactivity was also observed in neuronal growth cones and in some fibres running along blood vessels. In conclusion, GABA immunoreactive fibres appear to reach the cochlea by two routes: via the intraganglionic bundle and to a much lesser extent via the central bundles of the spiral ganglion. The fibres innervate sensory cells and also some spiral neurons. The occasional presence of GABA-positive neurons in the vestibular ganglion, in the VIIIth nerve nucleus, and exceptionally among the spiral neurons raises the possibility of a local GABAergic circuitry within the inner ear.