The properties of single chloride channels activated by gamma-aminobutyric acid (GABA) were investigated with hippocampal slices from adult guinea pigs. After the slices were treated with proteolytic enzymes, gigaseal recordings were made from excised patches of pyramidal or granule cell membranes. This newly developed preparation permits the application of patch-clamp techniques to the adult mammalian central nervous system. Guinea pig hippocampal slices were prepared in a conventional manner. Once prepared, the slices were treated with two different enzymes for brief periods and gently agitated. Slices generally split apart along the boundaries of the cell body regions, exposing neuronal somata. Standard patch-clamp techniques were used for the gigaseal recordings from excised patches. Solutions for both sides of the patches consisted of symmetrical concentrations of chloride, with all cation channels blocked. GABA at concentrations of 0.5-1.0 microM was added to the solution for the extracellular side of the patches. At transmembrane potentials negative to the chloride reversal potential (0 mV), the conductance through the GABA-gated chloride channels was approximately 20 pS. When the transmembrane potential was changed to positive values, the chloride conductance increased dramatically. For example, at +40 mV the conductance through the GABA-gated channels was almost 40 pS. Ramp-clamp commands were used to measure the current-voltage (I-V) relationship through single open channels. The open-channel I-V curves displayed outward rectification. The relationship between open-channel conductance and voltage could be fitted reasonably well by a single energy-barrier model for the channel, with the higher energy barrier placed near the cytoplasmic side of the membrane (at a fractional distance through the membrane of 0.73).(ABSTRACT TRUNCATED AT 250 WORDS)