Length- and width-summation curves of striate cortex cells revealed that there exist facilitatory, inhibitory or disinhibitory integration fields (IF) beyond the sides and ends of the classical receptive field (RF). The extent of the IFs is most frequently 2-5 times the size of the RFs. The tuning properties of IFs were studied using an annular surround grating patch while an optimal centre patch was placed at the excitatory RF to continuously activate the cell. The results show that, for most cells, the orientation, spatial frequency and speed tuning of the IFs were similar to, but broader than, the tuning of the RF, whereas the direction selectivity of the IF was not as pronounced as that of the RF. The possible functional significance of the IF is discussed.