Intracellular recordings have been made from myotomal motoneurones during "fictive swimming" in the in vitro preparation of the lamprey spinal cord, while monitoring the efferent burst activity in the ventral roots. The pattern of rhythmic activity in the motoneurones is described, as well as how synaptic inputs from the premotoneuronal level exert their control of motoneurone activity. (1) All motoneurones investigated displayed rhythmic, symmetric oscillations of their membrane potential during "fictive swimming". The period of depolarization occurred in phase with the burst discharge in the ventral root containing the motoneurone axon. (2) About one-third of the cells fired bursts of action potentials during the depolarized phase, while the remaining motoneurones exhibited subthreshold oscillations. (3) Intracellular injection of chloride ions reversed the sign of the hyperpolarized phase, demonstrating phasic active inhibition of the motoneurones during rhythmicity. (4) The depolarized phase was unaffected after chloride injection, showing that the motoneurones also received phasic active excitation. (5) "Pre-triggered" averaging of the motoneurone recording (using the ventral root spikes from other motoneurones for triggering), revealed that some degree of synchronous excitation of several motoneurones occurred, suggesting common excitation from the same premotor-interneurones. It is concluded that the rhythmic oscillations of membrane potential in lamprey myotomal motoneurones during "fictive locomotion" depend on phasic excitation alternating with phasic active inhibition. The premotoneuronal mechanism responsible for this control may consist of reciprocally organized groups of excitatory and inhibitory interneurones.