The purpose of the study was to measure nasal passageway dimensions in guinea pigs and rats by use of acoustic rhinometry (AR) and by a previously described fluid-displacement-method (FDM) (J Appl Physiol. 95:635-642, 2003) to investigate the potential of AR in pharmacological research with these animals. We measured the area-distance relationships by AR of nasal cavities post mortem in 5 guinea pigs (Duncan Hartley, 400 g) and 5 rats (Wistar, 250 g) using custom made equipment scaled for the purpose. Nosepieces were made from plastic pipette tips and either inserted into or glued onto the nostrils. We used liquid perfluorocarbon in the fluid-displacement study, and it was carried out subsequent to the acoustic measurements. We found for guinea pigs that AR measured a mean volume of 98 mm³ (95-100) (mean and 95% confidence interval) of the first 2 cm of the cavity. FDM measured a mean volume of 146 mm³ (117-175), meaning AR only measured 70% (50-90) of the volume by FDM. For rats the volume from 0 to 2 cm was 58 mm³ (55-61) by AR and 73 mm³ (60-87) by FDM, resulting in AR only measuring 83% (66-100) of volume by FDM (table 2). We conclude that absolute nasal cavity dimensions are underestimated by AR in guinea pigs and rats. This does not preclude that relative changes may be correctly measured. In vivo trials with AR using rats have not yet been published. The fluid-displacement-method is possibly the most accurate alternative to acoustic rhinometry for measurements of the nasal cavity geometry in small laboratory animals, but can only be used post mortem.