1. Receptive fields of simple cells in area 17 of the cat were mapped with stationary stimuli. Spatial frequency selectivities of the same cells were measured with drifting sinusoidal gratings. 2. The reconstructed field profile (inverse Fourier transform of selectivity curve) shows qualitative agreement with the mapped profile, and suggests the existence of additional side-lobes in the field. The side-lobes may correspond to the 'unresponsive regions' investigated by Maffei & Fiorentini (1976). 3. Our data suggest that the simple cell may perform approximately linear spatial summation of inputs to the visual system. However, the output of the simple cell is generally non-linear as reflected in its truncated responses to gratings.