The synaptic connectivity between rod bipolar cells and GABAergic neurons in the inner plexiform layer (IPL) of the rat retina was studied using two immunocytochemical markers. Rod bipolar cells were stained with an antibody specific for protein kinase C (PKC, alpha isoenzyme), and GABAergic neurons were stained with an antiserum specific for glutamic-acid decarboxylase (GAD). Some amacrine cells were also labeled with the anti-PKC antiserum. All PKC-labeled amacrine cells examined showed GABA immunoreactivity, indicating that PKC-labeled amacrine cells constitute a subpopulation of GABAergic amacrine cells in the rat retina. A total of 150 ribbon synapses established by rod bipolar cells were observed in the IPL. One member of the postsynaptic dyads was always an unlabeled AII amacrine cell process, and the other belonged to an amacrine-cell process showing GAD immunoreactivity. The majority (n=92) (61.3%) of these processes made reciprocal synapses back to the axon terminals of rod bipolar cells. In addition, 78 conventional synapses onto rod bipolar axons were observed, and among them 52 (66.7%) were GAD-immunoreactive. Thus GABA provides the major inhibitory input to rod bipolar cells.