The regional variation of contrast sensitivity along the greater extent of each of the four principal hemi-meridia of the normal human eye was determined under photopic conditions using horizontally-orientated sinusoidal grating stimuli. The stimuli were well localized in space and frequency, and special attention was paid to the low spatial frequency range. The results confirm that contrast sensitivity is maximal for central vision for all test spatial stimuli. Extra-foveal fall-off in sensitivity can be represented as a linear function of eccentricity if the latter is expressed in relative units (i.e. periods of the stimulus). The regional variation parameter depends upon whether the horizontal or vertical field is tested and upon the spatial frequency of stimulation. The visible spatial frequency range (0.05-24 c/deg) can be approximately described by just three different rules. The fact that more than one rule is found bears upon current models of the functional organization of the visual system.