The spatial-frequency characteristics of the visual system were studied in mentally healthy persons and in patients with schizophrenia using a test of contrast sensivity in a comparison of the contrast of two gratings with sinusoidal distribution of brightness and with the low, medium, or high spatial frequencies, which are perceived by neurons of the magnocellular and parvocellular channels with different sensitivities. In the first-episode schizophrenic patients who had received no antipsychotic drugs for a long time, an enhancement of contrast sensitivity as compared to mentally healthy subjects was observed when the patients were presented Gabor gratings with low spatial frequencies to which the magnocellular channel neurons are most sensitive. On the contrary, in the case of the contrast comparison of the gratings with the medium and high spatial frequencies, the contrast sensitivity was reduced in the first-episode schizophrenic patients irrespective of whether they had been non-treated or treated for a long time. In chronic patients with schizophrenia, reduced contrast sensitivity was observed in response to gratings with any frequency range tested. Some additional evidence of the internal noise enhancement in patients with schizophrenia has been also obtained. Our results make it possible to explain clinical data on specific disorders of visual perception at different stages of schizophrenia.