Nineteen rats were perfused intracardially with a 2% glutaraldehyde solution in cacodylate buffers adjusted in molarity from 0 to 0.4 M. Ultrathin sections of the inferior alveolar nerve were photographed in the electron microscope. The circumference, a shape factor, small diameter and myelin sheath thickness of each myelinated nerve fibre were measured using a semi-automatic image analysis system. Statistical analysis of the data revealed that the nerve profiles increasingly deviate from a true circle with higher concentrations of buffer. The small diameter of the myelinated nerve fibres declines linearly with increasing buffer molarity whereas circumference is unaffected. Myelin sheath thickness is correlated with fibre size but is not affected by changes in buffer molarity. The use of fibre circumference is recommended to allow valid comparison of results between studies in which fixation protocols may differ.