We previously reported that two types of visual thalamic interneurons (class III and class V) could be distinguished by the presence or absence of the enzyme brain nitric oxide synthase (bNOS; Bickford et al., 1999). In the present study, we found that further immunocytochemical characterization can be used to isolate class V and class III neurons: class V neurons express GABA and bNOS but not the calcium binding protein calbindin, whereas class III neurons express GABA and calbindin, but not bNOS. By pairing staining for bNOS or calbindin with staining for GABA, we were able to distinguish class III and class V neurons, thereby allowing us to characterize the synaptic inputs to these two interneuron populations within the cat pulvinar nucleus. We found class III and class V interneurons participate in distinctly different circuitries. Class III cells receive the majority of their input from large terminals with round vesicles (RL profiles, that presumably originate from cortical layer V) while class V cells receive all of their input from small terminals with round vesicles (RS profiles, that presumably originate from cortical layer VI). These two types of interneurons also target different regions of the dendritic arbors of thalamocortical cells; class III cells contact large caliber (presumably proximal) thalamocortical dendrites within glomeruli, while class V cells contact the small caliber (presumably distal) thalamocortical cell dendrites within the extraglomerular neuropil. Thus, a dual inhibitory system exists within the visual thalamus that integrates different types of corticothalamic input and targets distinct regions of thalamocortical cell dendritic arbors.