We have found high-affinity binding (site-A) and low-affinity binding (site-B) of benzodiazepines to membrane homogenates of embryonic chick brain and spinal cord. A new technique was developed to permit the determination of complete electrophysiological dose-response curves on single neurons in cell culture, eliminating cell-to-cell variability as a problem that complicates the interpretation of pooled data. The electrophysiological potencies and binding affinities of a series of benzodiazepines correlate well for site-A but not for site-B or the micromolar site reported in adult rat brain. Site-A and the electrophysiological response are sensitive to photo-affinity blockade with flunitrazepam (FNZM) by about 75% while site-B is resistant to blockade. The FNZM-photolinked benzodiazepine receptor/GABA receptor complex is not chronically potentiated and thus exists in an 'unpotentiated' state. These experiments suggest that site-A in embryonic CNS membranes corresponds to a functional benzodiazepine receptor/GABA receptor complex in spinal cord cell cultures.