An in vitro lamprey nervous system preparation has been developed which consists of the head and exposed brainstem attached to the isolated spinal cord (resting on the notochord). Mechanical or electrical stimulation of the snout elicits motor activity in ventral roots which underlies a turning response (head withdrawal) away from the stimulus followed by escape swimming. Direct stimulation of the sensory division of the trigeminal nerve activates these patterns, and cutting this nerve abolishes ventral root activity elicited by stimulation of the snout. These patterns of ventral root activity are correlated with muscle activity and escape movements in intact animals. Sensory input activated by passive bending of the notochord/spinal cord gates the first burst in ipsilateral ventral roots during turning motor activity, and this response can thus be considered as a position dependent 'enhancement' reflex. Descending pathways activated by stimulation of the snout consist of axons which project for at least 20 segments, and are not significantly dependent on propagation through local circuits in the gray matter. The in vitro brainstem/spinal cord preparation survives for several days and will permit studies of the descending systems which normally initiate two types of motor acts, swimming and turning responses.