The main function of postural nervous mechanisms in different species, from mollusc to man, is to counteract the force of gravity and stabilize body orientation in space. Here we investigate the basic principles of postural control in a simple animal model, the marine mollusc Clione limacina. When swimming, C. limacina maintains its vertical orientation because of the activity of the postural neuronal network. Driven by gravity-sensing organs (statocysts), the network causes postural corrections by producing tail flexions. To understand how this function occurs, we studied network activity by using a new method. We used an in vitro preparation that consisted of the central nervous system isolated with the statocysts. Output signals from the network (electrical activity of tail motor neurons) controlled an electrical motor which rotated the preparation in space. We analysed the activity of individual neurons involved in postural stabilization under opened or closed feedback loop. When we closed this artificial feedback loop, the network stabilized the vertical orientation of the preparation. This stabilization is based on the tendency of the network to minimize the difference between the activities of the two antagonistic groups of neurons, which are driven by orientation-dependent sensory inputs.