This study examines whether there is a relationship between the abundance of expression for P0 mRNA in myelinated Schwann cells and fibre diameter. Individual teased sciatic nerve fibres from young adult rats were hybridized with radiolabelled probe for P0 mRNA which is expressed in the perinuclear cytoplasm of the mid-internode. Signal intensity was measured as optical density of the developed autoradiograms. A highly significant positive linear correlation was present between signal intensity and fibre diameter. In a companion study, individual fibres were mounted in Araldite resin and transversely serially sectioned at 4 microns for autoradiography. Grain densities were determined for fibres of different diameters. Again, larger diameter fibres were associated with higher grain densities. The results indicate that the abundance of P0 mRNA expressed by a myelin-producing Schwann cell is related to fibre diameter with axonal size probably being the critical determinant. Axons may regulate P0 expression through the number of signalling molecules exposed on or released from the axolemma.