The mucous membranes of the accessory nasal sinuses and of the mastoid labyrinth were measured in undecalcified, plastic-embedded specimens from nonpathologic postmortem cases. Epithelial thicknesses, lamina propria thicknesses and the ratio of nuclear to total cell area of the epithelia are presented for these organs to provide a quantitative basis for dosimetric calculations of alpha-emitting radionuclides. These data will be of value for studies in populations at risk for head carcinomas from the intake of radium and related nuclides.