We were able to train cats raised with sutured eyelids to perform simple brightness discriminations before their lids were parted. If, and only if, a small hole was present in a lid, could some of the cats also perform a grating orientation discrimination. By establishing their thresholds for the brightness discrimination before and after dark adaptation and before and after the lids were opened, we reached three main conclusions. (1) During dark adaptation (with pupils maximally dilated and retinae most sensitive, regardless of lid suture), the cats were 3-4 log units more sensitive with the lids open than with the lids closed. This indicates a 3-4 log unit attenuation for the lids which is in agreement with our photometric measurements. (2) During light adaptation, the sensitivity difference between the conditions of opened and closed lids was only 1-2 log units. We concluded that factors (such as pupil dilatation and retinal sensitivity) partially compensated for the lid attenuation, since the open eye could have a smaller pupil and less sensitive retina during light adaptation. (3) Given these potential compensatory features of the pupil and assuming consensual pupil sizes, the deprived eye of a monocularly sutured cat may suffer more photic deprivation (since the pupil behind the closed lid would be as constricted as the pupil in the open eye) than would either eye of a binocularly sutured cat (where both pupils can be relatively large).