Modulation of responses elicited by moving bars within the classical receptive fields (CRF) of cat area 17 neurons were studied as a function of the direction and velocity of drifting gratings in the surrounds. Several different types of modulation were observed; collectively, the responses of most cells, both simple and complex, were strongly modulated by surround motion. None of these cells appear to signal relative velocity between the CRF and its surround. The gain and spatiotemporal structure of the CRF mechanism were estimated using contrast-response functions and reverse correlation with spatiotemporal ternary white noise, respectively. These measurements were made in the presence of surround gratings shown to significantly modify responses elicited from the CRF. In all cases, the gain of the CRF mechanism was driven up or down relative to controls but the receptive-field structure did not change in any way. We conclude that neurons in cat area 17 act like scalable filters, meaning that their gains can be adjusted by stimuli in the surrounds without altering the properties of the CRF. This was verified by showing that velocity tuning curves were also unmodified by stimuli in the surround that did change the gain. Based in part on these data, we discuss the notion that primary visual cortex makes use of a double-opponent mechanism for the representation of local discontinuities in motion and orientation.