AbstractThis paper describes the results of intracellular injections of radiolabelled neurotransmitters and transmitter precursor substances, including glutamate, GABA, aspartate, octopamine, tyramine, tryptophan, and choline, into cell bodies of identified excitatory and inhibitory neurons innervating lobster extensor musculature. The distributions and identities of radioactive substances appearing in axons were examined at various times following injection andin vitroincubation. Injected GABA and glutamate were found in appreciable quantities in both excitatory and inhibitory axons and migrated down axons at an estimated rate of between 16 and 22 mm/day at 12° C, whereas the other substances tested were present in substantially smaller quantities and migrated at an estimated rate of less than 7.5 mm/day at 12° C. Injected GABA,D‐glutamate andL‐glutamate accumulated proximal to ligatures tied around nerves, whereas neither octopamine nor aspartate accumulated proximal to ligatures. Since GABA is the transmitter substance released by inhibitory neurons andL‐glutamate is thought to be released from excitatory nerve terminals, these results are consistent with the suggestion that amino acids serving as neurotransmitters are axonally transported. The specificity of axonal transport does not appear to be restricted to the cognate neurotransmitter, as indicated by the movement of L‐glutamate in inhibitory axons and GABA in excitatory axons and ofD‐glutamate in both excitatory and inhibitory axons, but rather may be relaxed to include substances closely related to the neurotransmitter. Some restrictions, however, are apparently placed on axonal transport of small chaged molecules in these neurons in that other substances tested migrated down nerves at a considerably