The binocular summation effect of human VECP (increase in amplitude from about 4.0 microV to about 5.4 microV) was proved and optimized in 32 subjects. Stimulus parameters: TV steady-state pattern reversal (7.0 Hz); pattern size 1 degree; stimulus contrast 5% (higher stimulus contrasts reduced considerably the selectivity of the binocular summation effect); 96 sweeps averaging (Nicolet Compaq Four); position of electrodes: 10% and 30% above Protuberantia occipitalis externa of nasioninion distance. Aniseikonia of 7 to 52% was generated by means of small Galilei telescopes. Because of comparatively large interindividual variance within the series of subjects, no decrease in binocular VECP amplitude could be demonstrated at an aniseikonia less than 52%. On one woman subject, a significant reduction in amplitude (5% level) at 14%, 26% and 52% aniseikonia was apparent, but not at 7%. Thus, for this subject, it was possible to correlate the results with the clinically known limits of toleration for aniseikonia. This result was confirmed by more than 60% of the subjects in the test group.