Abstract Inhibitory neurons of the substantia nigra pars reticulata (SNr) serve as a primary output through which the basal ganglia regulate behavior. Using a virally targeted optogenetic approach, combined with whole cell patch-clamp recordings of SNr neurons, we show that, in mice, projection neurons of both primary and secondary motor cortices (M1 and M2) form monosynaptic excitatory connections onto different subpopulations of GABAergic SNr neurons.