Neurones of foetal mouse spinal cord or brain were grown dissociated in tissue culture. Intracellular recordings from single neurones demonstrated two types of conductance changes in response to applications of L-glutamic acid and a number of structurally related compounds. In the presence of 10 mM MgCl2 the responses consisted of an increase in membrane conductance. However, if this divalent cation was kept at ambient concentration a decrease of membrane conductance was also noted (perhaps to potassium ions). Often this decrease in conductance was associated with the appearance of regenerative and spikelike potentials even in the presence of tetrodotoxin. All amino acids tested with the exception of DL-kainic acid produced such a change in conductance. It is concluded that at least two distinct conductance mechanisms are linked to excitatory amino acid receptors.