Voltage-clamp recordings from mouse spinal neurons grown in culture were used to study the membrane current fluctuations induced by 12 substances structurally similar to gamma-aminobutyric acid (GABA). Fluctuation analysis provided estimates of the electrical properties of the elementary events underlying these responses. Estimates of the mean conductance of channels activated by all of the substances except glycine did not differ significantly from that estimated for GABA, whereas mean durations of agonist-activated channels all differed significantly from that found for GABA. The results indicate that all of the substances tested except glycine activate channels of similar conductance but of different durations.