1. Combined recording or ionophoretic electrodes of the concentric type were used to investigate the depolarizing responses of DL-homocysteate (DLH) and L-glutamate in cat lumbar motoneurones. 2. Typically, DLH responses were slow both in onset and recovery, while glutamate responses were fast in onset and recovery and were frequently accompanied by a post-response hyperpolarization. 3. DLH responses (smaller than those necessary to evoke firing) were accompanied by a stable decrease in GM. This decrease was usually more than could be accounted for by anomalous rectification of the membrane. 4. Small glutamate responses were accompanied by either a small decrease, no change or a small increase in GM. There was a biphasic change in GM during large responses: GM decreased during the rising phase and early part of the response plateau and thereafter increased as the depolarization was maintained. It is proposed that the high conductance state during glutamate application (but not the depolarization itself) is a manifestation of glutamate uptake. 5. Firing evoked by DLH was stable during very long applications of the drug. Firing evoked by glutamate was usually of short duration, despite the maintained depolarization. 6. No reversal potential for the DLH responses could be demonstrated, but the responses decreased in size both with hyperpolarization and depolarization of the membrane. A 'null point' of the response in the negative direction was found to be approximately -95 mV. 7. DLH resonses were insensitive to changes in the internal Cl concentration. When the external K concentration was increased by K+ ionophoresis, the DLH responses became smaller. It is concluded that the DLH response is probably mediated via a decrease in K+ conductance and that the availability of this conductance channel is potential dependent. 8. Changes in the sizes of evoked potentials (e.p.s.p.s, i.p.s.p.s and a.h.p.s) with DLH and glutamate responses were investigated. The size of each of these evoked potentials was inversely related to GM during the responses; thus they all showed stable increases during DLH responses. E.p.s.p.s recorded during DLH were of longer half-width and time-to-peak than the control, but there was no change in the maximum slope (V.sec-1). When e.p.s.p.s decreased in size with glutamate the time-to-peak remained constant. 9. Acidic amino acids have been implicated as natural excitatory transmitters. The consequence of our results for the mechanism of excitatory transmission is therefore discussed.