While many types of experiments have confirmed that the superior colliculus contributes to the initiation of orienting movements,1-7 very little is known at the cellular level about how its intrinsic circuitry performs this function. We currently are addressing this question by using in vitro whole-cell patch-clamp methods to carry out anatomical and physiological analyses of collicular circuitry. The objective of this work is to understand how premotor cells of the intermediate gray layer integrate inputs from sources external to the superior colliculus with the activity of intrinsic collicular circuitry to generate command signals for orienting movements.8 During the past few years, we used this in vitro approach to demonstrate an excitatory columnar pathway from visuosensory cells of the superficial gray layer (SGS) to the premotor cells of the intermediate gray layer (SGI). We then analyzed the synaptic actions of this pathway using similar methods.