An early study by Keller reported that medial rectus motoneurons display a step change in firing rate during accommodative vergence movements. However, a later study by Mays and Porter reported gradual changes in firing rate during symmetrical vergence movements. Furthermore, subsequent inspection of the activity of individual medial rectus motoneurons during vergence movements indicated transient changes in their firing rate that had not been noted by Mays and Porter. For conjugate eye movements, in addition to a position signal, motoneurons display an eye velocity signal that compensates for the characteristics of the oculomotor plant. This suggested that the transient change in firing rate seen during vergence movements represented a velocity signal. Therefore the present study used single-unit recording techniques in alert rhesus monkeys to examine the dynamic behavior of medial rectus motoneurons during vergence eye movements. The relationship between firing rate and eye velocity was first studied for vergence responses to step changes in binocular disparity and accommodative demand. Inspection of single trials showed that medial rectus motoneurons display transient changes in firing rate during vergence eye movements. To better visualize the dynamic signal during vergence movements, an expected firing rate (eye position multiplied by position sensitivity of the cell plus its baseline firing rate) was subtracted from the actual firing rate to yield a difference firing rate, which was displayed along with the eye velocity trace for individual trials. During all smooth symmetrical vergence movements, the profile of the difference firing rate very closely resembled the velocity profile. To quantify the relationship between eye velocity and firing rate, two approaches were taken. In one, peak eye velocity was plotted against the difference firing rate. This plot yielded a measure of the velocity sensitivity of the cell (pr(v)). In the other, a scatter plot was produced in which horizontal eye velocity throughout the vergence eye movement was plotted against the difference firing rate. This plot yielded a second measure of the velocity sensitivity of the cell (r(v)). The behavior of 10 cells was studied during both sinusoidal vergence tracking and conjugate smooth pursuit over a range of frequencies from 0.125 to 1.0 Hz. This enabled the frequency sensitivity of the medial rectus motoneurons to be assessed for both types of movements. Both vergence velocity sensitivity and smooth pursuit velocity sensitivity decreased with increasing frequency. This is similar to a finding by Fuchs and co-workers for lateral rectus motoneurons during smooth pursuit eye movements. However, unlike Fuchs and co-workers, we observed no significant changes in position sensitivity as a function of frequency. In addition, there was no significant correlation between the time constants of the motoneurons for vergence and smooth pursuit at any of the frequencies studied. These results show that vergence eye movements are not produced by step changes in firing rate of medial rectus motoneurons. During vergence eye movements, medial rectus motoneurons display signals related not only to eye position, but to velocity and to higher order components just as they do for conjugate eye movements.