The immunoreactivity of the high-molecular-weight neurofilament (NF) subunit toward antibodies that react with phosphorylation-related epitopes was determined at different anatomic sites in the PNS of rats during normal maturation and after intoxication with beta,beta'-iminodipropionitrile (IDPN). A maturational increase in the relative binding of phosphorylation-dependent antibodies compared to phosphorylation-inhibited antibodies occurred from age 3 to 12 weeks. An increase in phosphorylation-related immunoreactivity with increasing distance from the cell bodies was present in ventral and dorsal roots at all ages. The degree of phosphorylation-related immunoreactivity was greater for centrally directed axons in the dorsal roots of the L5 ganglion than for peripherally directed axons. IDPN, a toxin that impairs NF transport, caused a marked increase in reactivity toward the phosphorylation-dependent antibody. NFs from IDPN-treated rats also bound less of an antibody that is normally phosphorylation independent and this inhibition of binding was sensitive to phosphatase digestion. In each instance, greater degrees of phosphorylation-dependent immunoreactivity correlate with conditions known to exhibit slower net rates of axonal transport of NF proteins.