Electrophysiological and morphological properties of premotor neurones in the vestibulo-ocular system have been studied intracellularly in the unanesthetized "encéphale isolé" cat. 1. Secondary neurones of the vestibulo-abducens reflex, identified both by their antidromic and orthodromic responses, showed no nystagmic modulation of their activity. 2. Only 12% of the recorded population in the medial vestibular nucleus showed burst firing patterns correlated with the phases of nystagmus. 3. Two classes of neurones which did not respond to stimulation of the abducens nerve were recorded in the abducens nucleus. The first were inhibited by phasic vestibular stimulation and fired 180 degrees out of phase with the ipsalateral motor nerve discharge. The second type of neurones was excited by vestibular stimulation and fired in synchrony with the motor nerve activity.