A novel tissue preparation technique for improving gamma-aminobutyric acid (GABA) immunocytochemistry has been developed. The influence of the glutaraldehyde concentration in the fixative and the effect of pronase treatment on the GABA immunostaining were tested. This method includes fixation with a high concentration of glutaraldehyde, gelatin embedding and treatment of the sections with pronase. In sympathetic (paravertebral) ganglia and their connectives, the most intense and specific immunoreaction was obtained with the following procedure: immersion fixation in 5% glutaraldehyde, infiltration and embedding in 15% gelatin, secondary fixation of the samples with 4% formaldehyde, floating frozen sections and digestion with 0.1% pronase for 15-20 min. With this technique, the GABA-containing structures (cells and nerve fibers with varicosities forming basket-like networks around some principal neurons) were selectively labeled. The data presented suggest that (1) a high concentration (5%) of glutaraldehyde in the primary fixative is necessary to preserve a large proportion of the GABA content; (2) this glutaraldehyde fixation partly masks the GABA immunoreactivity; and (3) this masking may be overcome by a proteolytic treatment preceding the immunostaining. This method has been extensively tested for the light microscopic visualization of GABA-containing tissue components in the sympathetic ganglion chain, but it may probably also be used for the immunocytochemical detection of other small molecules in other parts of the nervous system.