1. Intracellular recordings were made from the cell bodies and axons of giant reticulospinal neurones (Müller cells) of the lamprey, and responses to bath- and ionophoretically applied glutamate and aspartate were studied. 2. Bath-applied glutamate and aspartate depolarized both cell bodies and axons, but there appeared to be an associated conductance increase only in the cell bodies. The depolarization of Müller axons by the bath-applied drugs probably resulted from the passive flow of current into them from spinal cells to which the axons are coupled electrically. 3. The reversal potentials for responses to ionophoretically applied glutamate and for excitatory post-synaptic potentials (e.p.s.p.s) evoked by stimulation of the contralateral vestibular nerve were directly determined in Müller cell bodies which had been damaged by penetration with low-resistance electrodes. The glutamate and e.p.s.p. reversal potentials were identical, the average difference in eight cells being 0.31 mV. The absolute value of the e.p.s.p.--glutamate reversal potential varied from --16 to --35 mV in different cells, with the more negative values occurring in less damaged cells with higher resting potentials. 4. Injection of Cl into Müller cell bodies had no effect on the e.p.s.p.--glutamate reversal potential. Reduction of the extracellular Na concentration to 1 over 10 normal produced a negative shift in the glutamate reversal potential. 5. It is proposed that the natural excitatory transmitter and glutamate produce identical conductance changes in Müller cells, involving an increase in Na and K conductance.