This study was designed to test the hypothesis that microcysts in the gerbil auditory system are formed from neuronal somata. Six neuronal types (octopus, multipolar, bushy, elongate, miscellaneous and small) were distinguished, counted and measured along with the microcysts in the posteroventral cochlear nuclei (PVCNs) of 3, 12 and 36 month old gerbils. No decrease was observed in the numbers of neurons in any neuronal class, or in the neuronal population as a whole, in the PVCN of the gerbil as a function of age. Neither was any change observed in the PVCN area occupied by non-neuronal, non-microcyst elements. Neuronal sizes were unchanged between 3 and 12 months, but multipolar and bushy cells, as well as the total neuronal population decreased significantly in size between 12 and 36 months. The number and size of microcysts increased significantly between 3 and 12 months of age and accounts for increases in PVCN volume. The number and size of microcysts decreased significantly between 12 and 36 months. Thus, the appearance of microcysts can not result from the selective loss of any single class of neurons. Hearing was assessed in five 36 month old animals with auditory brainstem responses (ABR) and number and size of microcysts were found to correlate with hearing status, being largest and most numerous in animals with the best hearing, and smallest and fewest in the deaf animal. It is concluded that microcysts cannot represent a neurodegenerative disease of neuronal somata. Microcyst formation appears to be a dynamic process related to the degree of auditory stimulation.