120 cells were recorded from area 17 of cats and their length and width summation properties were investigated by drifting sinusoidal grating patches. The study shows that (1) for most of the cells, the RF was roughly equal in length and width, while most of the integration fields (IFs) were of elongated shape either in length or in width; (2) the size of the IFs was mostly 2-7 times (mean 3.7 times) the size of the related RFs, no significant difference was seen between the simple and the complex cells; (3) the size of the IFs tends to increase with increase of retinal eccentricity of the RFs; (4) for neurones whose RF located near the vertical meridian (VM), the IF extended bilaterally into both the contra- and the ipsilateral sides to about the same extent, but the strength of the modulatory effect of the ipsilateral field is weaker than that of the contralateral field. The above results indicate that neurones of the primary visual cortex integrate visual information from an extensive visual field, and the integration effect may cross the VM to the ipsilateral side.