The effects of different check sizes on the 100m component of pattern reversal visual evoked magnetic fields (VEF) and the P100 component of visual evoked potentials (VEP) in terms of latency, amplitude and source localization were analyzed. Half field stimuli with or without central occlusion with check sizes of 15', 30', 60', 90' and 180' of visual arc were given to 7 healthy subjects. VEF and VEP were recorded simultaneously. The effect of the check size on the peak latency of both 100m and P100 was significant (P<0.01, ANOVA). The latencies for the smaller checks were significantly longer than those for the larger checks. The effect of the check size on the amplitude of the 100m to the stimulation with central occlusion was significant (P<0.05, ANOVA), but was not to the stimulation without central occlusion. That is, the amplitudes for the smaller checks were significantly smaller than those for the larger checks when using the stimulation with central occlusion, but not the stimulation without central occlusion. The effect of the check size on the P100 amplitude was not significant to the stimulation with and without central occlusion. The equivalent current dipoles were located around the calcarine fissure and did not differ significantly in location with check size. In conclusion, check size significantly affects the latency and amplitude of the 100m and/or P100, but not the receptive areas for the stimulation.