Primary cultured sensory neurons prepared from adult mice were maintained for 8 days in vitro. Such cultures were exposed to either a range of ethanol concentrations (50-300 mM) or acetaldehyde (0.5-2 mM) in serum-free medium for up to 24 h. Treated neuronal cultures, together with untreated controls in both the presence and absence of serum, were prepared for transmission electron microscopy. Nuclear morphology was not changed following treatment with either substance at the doses studied. A number of changes were observed, however, in the cytoplasm of neurons, and these were intensified by an increase in concentration and the length of exposure. Acetaldehyde induced effects at a much lower concentration than was required to induce a response with ethanol. Myelin lamellae loosely wound around dense granular core material appeared in multivesicular bodies at low doses. The prevalence of these increased with concentrations of 100 mM ethanol and 1 mM acetaldehyde; the numbers of lamellae in each myelin figure also increased but the core material was less prominent. Electron-dense bodies were also evident at higher dosages together with evidence of vacuolation of the endoplasmic reticulum and Golgi complexes. Mitochondrial profiles similar to those in untreated neurons persisted throughout the exposure periods. The generation of these inclusions may reflect a mechanism of membrane turnover, both of internal systems and cell membrane cycling, as a response to alcohol and aldehyde treatment.