1. We have examined the spatial and temporal tuning properties of 238 cortical neurones, recorded using conventional techniques from acutely prepared anaesthetized cats. We determined spatial and temporal frequency tuning curves using sinusoidal grating stimuli presented to each neurone's receptive field by a digital computer on a cathode ray tube. 2. We measured tuning curves either by determining response amplitude as a function of spatial or temporal frequency, or by measuring contrast sensitivity (the inverse of the contrast of the grating that just elicited a detectable response). The two measures give very similar tuning curves in all cases. 3. We recorded from 184 neurones in area 17; of these 156 had receptive fields within 5 degrees of the area centralis. The range of preferred spatial frequency for these neurones was 0.3--3 c/deg, and their spatial frequency tuning band widths varied from 0.7 to 3.2 octaves at half-amplitude. The most common band width was roughly 1.3 octaves. Simple and complex cells in area 17 did not differ in their distributions of preferred spatial frequency, although complex cells were, on average, slightly less selective for spatial frequency than simple cells. 4. We recorded from fifty-four neurones from area 18, and performed several experiments in which we recorded from corresponding portions of both area 17 and area 18 in the same electrode penetration. Neurones in area 18 preferred spatial frequencies that were, on average, one third as high as those preferred by area 17 neurones at the same retinal eccentricity. Thus the range of preferred spatial frequency in area eighteen cells having receptive fields within 5 deg of the area centralis was between less than 0.1 and 0.5 c/deg. The distributions of optimum spatial frequency in the two areas were practically non-overlapping at eccentricities as high as 15 deg, the greatest eccentricity we examined. Neurones in area 18 were about as selective for spatial frequency as were neurones in area 17. 5. We determined temporal frequency tuning characteristics for some neurones from each area, using gratings that moved steadily across the screen. Neurones from area 17 all responded well to low temporal frequencies, and less well to higher frequencies (in excess of, usually, 2 or 4 Hz). In contrast, neurones recorded from area 18 sometimes had similar tuning properties, but more commonly showed a pronounced reduction in response as the temporal frequency was moved either above or below some optimum value (usually 2--8 Hz). 6. We conclude from these results that areas 17 and 18 act in parallel to process different aspects of the visual information relayed from the retina via the lateral geniculate complex. Some or all of the differences between the areas may be attributable to the predominance of Y cell input to area 18 and the predominance of X cell input to area 17...