This study is concerned with the quantification of changes which occur in peripheral nerves of normal mice from the onset of maturity to old age. The parameters chosen were total fibre area and axon area. Size distributions of these were expressed in the form of histograms. From the area data notional fibre and axon diameters were calculated, together with the thickness of the myelin sheath. The notional diameters were contrasted with the comparable myelin sheath thickness in the form of scatter diagrams. These results are intended to provide a reference scale for subsequent assessment of changes induced by drugs administered throughout life. The are in general accord with previous observations, but were obtained quickly, and with less margins of error, by using a Quantimet 720.