The distribution of the various epithelial types lining the nasal cavity in normal 7 and 16 weeks old male Fischer-344 rats and male B6C3F1 mice has been mapped at the light microscopic level. Photographs of transverse sections of the nose were analysed using a Zeiss Videoplan computerized image analysis system programmed for measurement and evaluation of count, area, perimeter and length. In rats, the volumes of the nasal cavity at 7 and 16 weeks are 156 and 257 mm3 respectively; while in mice the nasal cavity volume is essentially the same (32 . 5 and 31 . 5 mm3) at the same two ages. Total surface areas of the nasal cavity in rats at 7 and 16 weeks are 799 and 1344 mm2 respectively; and in mice 278 and 289 mm2. The percentages of the nasal cavity surface lined by squamous, respiratory and olfactory epithelium are similar at both ages in both species. Applications and significance of these data are discussed.