Excessive cortical excitation due to visual stimulation often leads to photosensitive epilepsy. Here we demonstrate that even in normal subjects, prolonged stimulation with low-luminance chromatic (equiluminant) flicker evokes neuromagnetic activity in the primary visual cortex, which develops slowly (up to 1000 ms) and depends on the color combination of flicker. This result suggests that chromatic sensitivity is a critical factor of cortical excitation, which can be amplified over time by a flickering stimulus. We further show that transient activity occurs in the parieto-occipital sulcus as early as 100-400 ms after flicker onset, which is negatively correlated with the later occipital activity. The early parieto-occipital activity may reflect a defensive mechanism that suppresses cortical hyperactivity due to chromatic flicker.