Morphometric evaluations were performed on the peroneal nerve from mutant quails deficient in neurofilaments (NF) to elucidate the effect of an absence of NF on unmyelinated axons. The diameter frequency distribution of unmyelinated axons was similar between controls and mutants. The mean transverse axonal area, axonal circumference and circularity index of the unmyelinated axons were also similar in controls and mutants. However, the number of microtubules (MT) per axon was greater (P < 0.01) in the mutants than in the controls. The regression analysis relating the number of MT per axon to the diameter of unmyelinated axons indicated a greater number of MT in the mutants than in the controls (P < 0.05-0.01). A significantly greater number of MT per axon in the mutants may suggest a compensatory increase of MT in the absence of NF. This may conserve the size and transverse circular profile of the unmyelinated axons which are probably maintained by both MT and NF in the controls. The number of MT may be increased at the expense of the soluble fraction of tubulin.