Transmission and scanning electron microscopic examinations were undertaken to detail changes in the olfactory epithelium (OE) resulting from the ageing process. Samples were prepared from 3% glutaraldehyde/1% formaldehyde perfused mice aged 6 months and 29-30 months. Compared to OE from young adults, a number of striking changes were apparent in tissue from older animals. The most obvious of these were extensive local accumulations of large inclusion bodies, totally disrupting the normal morphology of such affected areas of olfactory epithelium. Even in areas where these deposits were absent, other significant signs of ageing were noted in the seemingly unaffected OE of all older mice studied. Quantitative analysis of semi-thin resin sections revealed that the OE of aged mice was significantly reduced in thickness, and with significantly fewer olfactory sensory neurons, irrespective of whether or not inclusions were present. In addition, pale cells, which have been suggested to be a form of degenerating cells, were more abundant in aged OE. The straight, sharp boundary between respiratory and olfactory epithelia which is seen in young adults became irregular and disrupted with age due to an intermingling of the respiratory and olfactory cells. Such structural alterations may parallel olfactory dysfunction reputed to occur in older mammals.