1. Monkeys were trained to perform a variety of horizontal eye tracking tasks designed to reveal possible eye movement and vestibular sensitivities of neurons in the medulla. To test eye movement sensitivity, we required stationary monkeys to track a small spot that moved horizontally. To test vestibular sensitivity, we rotated the monkeys about a vertical axis and required them to fixate a target rotating with them to suppress the vestibuloocular reflex (VOR). 2. All of the 100 units described in our study were recorded from regions of the medulla that were prominently labeled after injections of horseradish peroxidase into the abducens nucleus. These regions include the nucleus prepositus hypoglossi (NPH), the medial vestibular nucleus (MVN), and their common border (the "marginal zone"). We report here the activities of three different types of neurons recorded in these regions. 3. Two types responded only during eye movements per se. Their firing rates increased with eye position; 86% had ipsilateral "on" directions. Almost three quarters (73%) of these medullary neurons exhibited a burst-tonic discharge pattern that is qualitatively similar to that of abducens motoneurons. There were, however, quantitative differences in that these medullary burst-position neurons were less sensitive to eye position than were abducens motoneurons and often did not pause completely for saccades in the off direction. The burst of medullary burst position neurons preceded the saccade by an average of 7.6 +/- 1.7 (SD) ms and, on average, lasted the duration of the saccade. The number of spikes in the burst was well correlated with saccade size. The second type of eye movement neuron displayed either no discernible burst or an inconsistent one for on-direction saccades and will be referred to as medullary position neurons. Neither the burst-position nor the position neurons responded when the animals suppressed the VOR; hence, they displayed no vestibular sensitivity. 4. The third type of neuron was sensitive to both eye movement and vestibular stimulation. These neurons increased their firing rates during horizontal head rotation and smooth pursuit eye movements in the same direction; most (76%) preferred ipsilateral head and eye movements. Their firing rates were approximately in phase with eye velocity during sinusoidal smooth pursuit and with head velocity during VOR suppression; on average, their eye velocity sensitivity was 50% greater than their vestibular sensitivity. Sixty percent of these eye/head velocity cells were also sensitive to eye position. 5. The NPH/MVN region contains many neurons that could provide an eye position signal to abducens neurons.(ABSTRACT TRUNCATED AT 400 WORDS)