Apparent contrast of a vertical sinusoidal grating in the simultaneous presence of peripheral gratings was measured as a function of peripheral contrast, with test contrast, and relative phase and position of the two gratings as parameters. When the peripheral gratings were horizontally adjacent to the test grating, irrespective of the phase relation, the apparent contrast was raised in the range of peripheral contrast below the test contrast, but depressed in the range of peripheral contrast above the test contrast. When the peripheral gratings were vertically adjacent to the test grating, a similar tendency as mentioned above was observed under the in-phase condition. Under the opposite-phase condition, the apparent contrast was raised monotonically with an increase in peripheral contrast. These effects can be explained in terms of three processes of brightness induction, spatial summation and interaction between the spatial-frequency selective mechanisms.