Sciatic nerves from rats were examined electron microscopically following fixation in 4% tannic acid in 2.5% glutaraldehyde, which allowed demonstration of a filamentous network between the usual intra-axonal organelles. The network appears to consist of longitudinal 10 nm in diameter filaments and cross-linking filaments of about 6 nm diameter. Exposure to cold caused disruption of microtubules, but not the filaments, and incubation at 37 degrees C following cold exposure resulted in reformation of the microtubules which again showed linking with the filaments. Exposure of the nerves to cold in the presence of D2O did not cause disruption of the microtubules but there did appear to be some loss of the fine filaments. These findings suggest that the finer cross-linking filaments are of a different nature than the longitudinal 10 nm filaments, and that there is a dynamic relationship between these filaments and microtubules since the cross-linkages reappear following microtubule disruption and reformation.