The relationship between the density of hair cells (cells/mm) and measured distance along the guinea pig organ of Corti was determined using light microscopy and the surface specimen technique. It was demonstrated that the density of inner hair cells (IHC; mean 92.0±2.4) and 1st row of outer hair cells (OHC1; mean 118.7±2.3) did not show significant variation along the organ of Corti except within 0.5–1.0 mm of the apex and base where there was considerable variation between animals in the density of cells. There was a close relationship between the accumulated number of either IHC or OHC1and distance from the base along the organ of Corti. Distances estimated by hair cell counts were similar to those determined by direct measurement. It is concluded that hair cell counts can be used to reliably estimate distances along the organ of Corti where accurate direct measurement is not possible, such as in scanning electron microscopy.