The activity of V1 neurons evoked by stimuli within the classical receptive field (CRF) is known to be modulated by stimuli in the surrounding field, the extra-receptive field (ERF). By varying the relative spatial phase (RSP) between a central grating presented in the CRF and a surround grating in the ERF, we studied the contextual modulation in V1 neurons of alert monkeys (Macaca mulata). Results from two monkeys show that most of the V1 neurons with suppressive ERF are sensitive to the RSP, and the degree of sensitivity is strongly dependent on the strength of ERF suppression. This sensitivity is maximal when the RSP is generated at or near CRF/ERF boundary, but is observed over the entire ERF. Interestingly, the suppressive effect of the surround grating can be largely abolished by inserting a narrow gap between the center and surround gratings or by a phase displacement between them corresponding to < 10% of the CRF diameter. These properties of V1 neurons may serve important perceptual functions.