GABA(C) receptors (GABA(C)Rs) are widely expressed in the mammalian subcortical visual system, particularly in the retina and superior colliculus (SC). GABA(C)Rs are composed of specific rho1-3 subunits the expression of which varies among visual structures. Thus rho1 subunits are most abundant in retina, and their loss eliminates GABA(C)R expression and function. In the SC, rho2 subunit expression may be equal to or stronger than rho1 subunit expression; however, results across studies vary considerably. To more directly assess the expression of GABA(C)R subunits, we characterized inhibition in the SC of wild-type (WT) and GABA(C) rho1 Null mice that lack expression of GABA(C) rho1 subunits. We used whole cell patch-clamp recordings and evaluated GABA(C)R-mediated modulation of electrically evoked post synaptic currents using either agonists or antagonists in WT mice. In GABA(C) rho1 Null stratum griseum superficiale (SGS) cells, inhibitory postsynaptic currents were shorter in duration and their excitatory postsynaptic currents (EPSCs) were longer, indicating that a slow GABA(C)R-mediated inhibitory component was reduced in each case. In contrast to retina, GABA(C)R-mediated currents in the SC were altered but not eliminated in GABA(C) rho1 Null mice. In the majority of SC cells in GABA(C) rho1 Null mice, GABA(C)R activation could still be induced to alter EPSC peak amplitudes in putative interneurons and in many projection neurons. These results, compared with previously published data, indicate a fundamental difference between retina and SC in the control of GABA(C)R expression and subunit composition.