Hearing loss in Varitint-waddler-J (VaJ) mice is of mixed origin with both cochleo-saccular and neuroepithelial components. Both VaJ/VaJ and VaJ/+ mutants show impaired cochlear function, but the homozygotes are more severely affected than heterozygotes. Neither group have any detectable compound action potential. Cochlear microphonics are only seen in half of the heterozygotes, at a reduced amplitude and raised threshold, and are not detected in any homozygotes. Summating potentials (SP) responses are seen in most of the heterozygotes, at high stimulus levels. The only responses in homozygotes were negative SPs seen in half of the mutants at very high sound levels, while the remaining homozygotes showed no responses to sound stimulation. Endocochlear potentials (EP) were often small or absent in both groups of mutants, with the homozygotes being more severely affected. Reduced pigmentation in the stria vascularis appears to be associated with a reduced EP, while a primary defect of the neuroepithelium, detectable by electron microscopy in hair cells of 14 day old mice, dramatically influences evoked potentials.