An examination of saccades during vergence eye movements (accommodative and disparity) reveals that binocular saccades have unequal magnitudes in each eye, the smaller saccade of each pair opposing the vergence movement. Furthermore, this inequality cannot be explained by linear summation of the saccadic amplitudes onto the ongoing vergence. Saccadic inequalities are usually greater at the start of the vergence; the amplitude of the smaller saccade may be reduced 70% from that of the larger. Using a sixth-order nonlinear model of the eye movement plant and linear summation of saccadic (pulse-step) and vergence (step) control signals to drive the plant, we have produced a reduction in saccadic amplitudes between the two eyes during vergence that is similar to those found in our subjects. These results suggest that the reduction of saccadic amplitudes is due to an interaction in the muscular system moving the eye, not to a nonlinear neuronal interaction more centrally located.