An EM-autoradiographical/immunocytochemical technique was used to study amacrine cell synapses onto mb1 bipolar cell terminals in goldfish retina. Tissue was double labeled for [3H]GABA uptake and glutamate decarboxylase (GAD) immunolocalization. Nearly 90% of the amacrine cell synaptic processes onto both proximal and distal halves of mb1 terminals were labeled with either [3H]GABA or GAD-immunoreactivity (IR). Proximal half: 73% of the amacrine synapses were labeled with [3H]GABA uptake and 82% with GAD-IR; 88% of [3H]GABA labeled contacts were double labeled. Distal half: 17% of the amacrine synapses were labeled with [3H]GABA uptake and 67% with GAD-IR; 63% of [3H]GABA labeled contacts were double labeled. After consideration of the possible sources of [3H]GABA labeled synapses onto mb1 terminals, we concluded that the synaptic terminals of pyriform Ab amacrine cells double label for [3H]GABA and GAD-IR despite our previous report that Ab cell bodies do not stain for anti-catfish brain GAD antiserum. We suggest that Ab cells contain isoenzymes of GAD which differ in subcellular distribution, thereby accounting for the differential staining of the cell bodies and dendrites obtained with the GAD antiserum we used.