Microtubule-associated proteins (MAPs), such as tau, modulate neuronal shape and process outgrowth by influencing the stability and organization of microtubules. The dynamic nature of MAP-microtubule interactions in vivo, however, is poorly understood. Here, we have assessed the stability of these interactions by investigating the synthesis and axoplasmic transport of tau in relation to that of tubulin and other MAPs within retinal ganglion cells of normal adult mice in vivo. Using immunoprecipitation and Western blot analysis with anti-tau monoclonal and polyclonal antibodies, we unequivocally identified in optic axons a family of 50–60 kDa tau isoforms and a second 90–95 KDa tau family, the members of which were shown to contain the domain of tau encoded by exon 4A. To measure the rates of translocation of tau proteins in vivo, we injected mice with 35S- methionine intravitreously and, after 6–30 d, quantitated the radiolabeled tau isoforms immunoprecipitated from eight consecutive 1.1 mm segments of the nerve and optic tract and separated by electrophoresis. Linear regression analysis of protein transport along optic axons showed that the tau isoforms advanced at a rate of 0.2–0.4 mm/d, and other radiolabeled MAPs, identified by their association with taxol-stabilized microtubules, moved three- to fivefold more rapidly. By contrast, tubulins advanced at 0.1–0.2 mm/d, significantly more slowly than tau or other MAPs. These studies establish that tau is not cotransported with tubulin or microtubules, indicating that associations of tau with microtubules within axons are not as stable as previously believed. Our findings also reveal differences among various MAPs in their interactions with microtubules and provide evidence that assembly and reorganization of the microtubule network is an active process even after axons establish connections and fully mature.