Development of the cholinergic enzymes, choline acetyltransferase (ChAT) and AChE, and of the AChE-positive innervation in the cochlea was studied biochemically and morphologically in the postnatal mouse up to 26 days. Both ChAT and AChE are already present at birth in levels comparable to 50 and 20% of near-adult values, respectively. Increases in the enzymatic activities occur mainly during the second postnatal week. ChAT increases primarily in the basal turn; the specific activities in the basal and mid turns become about equal and at least twice of the values found in the apex. AChE increase continues throughout the entire cochlea; at all times its activity is highest in the base and lowest in the apex. In the light microscope, AChE-positive fibres are seen to enter the organ in the intraganglionic bundle during late foetal development and travel upwards via radial bundles. The fibres destined for outer hair cells usually differentiate first and take a separate route. They either cross the prospective tunnel of Corti directly or take a spiral course in front of inner pillar cells to form the inner pillar bundle. The tunnel fibres are radially oriented and provide the innervation to outer hair cells in narrow vertical sectors. In most cases, the outer hair cells are being innervated by the 4th day. Between the 4th and the 6th day, the tunnel fibres reach the outer hair cells in the third row; the first and second outer spiral bundles are formed. The AChE-positive innervation of the inner spiral bundle and plexus forms in short segments, and the bundle may be still discontinuous even by the 6th day. By the 12th day the innervation is complete. In the electron microscope, the stain for AChE may allow identification of growing efferent fibres before their ultrastructural differentiation. Both ChAT and AChE activities are early markers of the differentiating efferent system. An ingrowth of the cholinergic fibres to the entire cochlea occurs before birth. The greatest increase of AChE occurs between the 4th and 10th day, relating in time to efferent synaptogenesis.