1. In precollicular decerebrate cats the electrical activity of single pontine neurons was recorded before, during and after the episodes of postural atonia produced by i.v. injection of 0.03-0.1 mg/kg of eserine sulphate. These episodes were characterized by the regular occurrence of horizontal conjugate eye movements, which were mainly grouped in bursts of REM; moreover, a burst of REM in one direction was generally followed by a burst of REM in the opposite direction. 2. Among the recorded units, 32 showed an increase in their discharge rate during these cataplectic episodes. However, while these units fired at regular frequency when postural rigidity was present, they showed periodic changes in their discharge rate as soon as the bursts of REM appeared in the electrooculogram. In particular a nearly sinusoidal increase in the discharge rate was related to the appearance of an ocular burst in one direction, while a decrease in the unit discharge occurred during an ocular burst in the opposite direction. In some instances neighbouring pontine units located within each side of the brain stem showed reciprocal rate profiles during REM bursts oriented in a given direction, making it likely that the cyclic alternation of their activity depended upon their reciprocal interaction. 3. The alternative hypothesis, i.e., that these periodic changes in unit discharge depend upon the proprioceptive feedback due to the eye movements was excluded by the fact that these changes started before the occurrence of the bursts of REM and began to decline before the end of the burst. Moreover no variation in their firing rate was observed during the positional nystagmus induced by tilting the animal in the control period, i.e., when postural rigidity had reappeared following the end of the cataplectic episode. 4. Most of the neurons showing periodic changes in their discharge frequency during the bursts of REM were located in the pontine reticular formation. Scattered units were also found within the region of the locus coeruleus and the raphe system, close to the surrounding reticular structures. 5. In addition to these neurons, 60 pontine units were recorded, which did not show any changes in their discharge rate during transition from the control period to the cataplectic episode. However, phsiic increases or phasic decreases in their discharge rate appeared synchronously with the individual eye movements. Since in most instances these phasic changes in unit activity coincided with the appearance of the individual monophasic potentials recorded from the ascending MLB, which immediately preceded the rapid eye movements, these units could be attributed either to the premotor neurons responsible for these REM or to the closely related structures which generate their rhythmic discharge. In only a few instances did the discharge of these units not precede but follow the individual eye movements, indicating that they resulted from a proprioceptive feedback originating during the eye movements. 6...