Intracellular recordings from in vitro neurones of the frog spinal cord slice preparation were performed in order to examine the mechanism of action of gamma-aminobutyrate and glutamate on two distinct neuronal populations in the same region of the central nervous system. Amino acids were superfused at fast rate and low temperature (7 degrees C) to reduce their uptake process. On interneurones, the inhibitory action of gamma-aminobutyrate was characterized by a large input conductance increase while on motoneurones the conductance change was much smaller. Glutamate excited interneurones which greatly increased their input conductance and showed burst firing; motoneurones were also excited by glutamate but usually did not fire repeatedly nor showed large conductance changes. In spite of these differences the amplitude of depolarization in the presence of the same concentration of glutamate was similar for motoneurones and interneurones. It is suggested that amino acids (particularly glutamate) may act through different membrane mechanisms on two neuronal populations in the same region of the spinal cord.