We have recently suggested (Redgrave et al 1997, 1998) that the basal ganglia (BG) play a crucial role in solving the action selection problem in vertebrates. We now present a quantitative model of intrinsic BG processing which shows how this might occur. We assume that several command systems with different levels of salience are requesting their actions to be enabled by the BG. Under dopaminergic control the striatum extracts the salience, and selects a subset of actions for subsequent processing by running a series of winner-take-all competitions mediated by short-range recurrent connections. Each such winning command system defines a channel of information flow through individual BG nuclei with the exception of the subthalamic nucleus (STN) which integrates information across channels. The model has been subject to an analysis of its equilibrium states and full dynamic computer simulation. There are three main results: (i) Diffuse excitation from STN and focused inhibition from striatu..