Summary Levels of protein and activity of several enzymes were determined in different portions of the visual system of the human brain. The enzymes studied were total cholinesterase (TChE), acetylcholinesterase (AChE), pseudocholinesterase (ChE), choline acetyltransferase (ChAc), monoamine oxidase (MAO), and N-methyltransferase (NMT). Brains of 17 patients were obtained at autopsy. The specimens were divided into 3 groups. One group consisted of the brains of subjects who were mentally normal prior to death. The second group of subjects had been diagnosed as chronic schizophrenics and the third had an organic brain syndrome. Each group included males and females. Subjects with an organic brain syndrome were significantly older than subjects in either of the other two groups. There were no significant differences in protein among the 3 groups for most of the visual areas. All enzyme activity was expressed per g of protein. TChE, AChE, and ChE activity varied markedly within the visual system, being usually lowest in the optic nerve and highest in the superior colliculus. There were no significant differences in TChE or AChE among the diagnostic groups. Unexpectedly, ChE was elevated in some brain areas in the chronic schizophrenic group. This finding needs further confirmation in a drug free group. ChAc did not differ significantly among the 3 diagnostic groups and, in general, was low throughout the visual system but highest in the superior colliculus and lateral geniculate. MAO activity was not significantly different among the 3 patient groups. Its activity in the gray matter was only slightly higher than in the white matter of the visual system. NMT activity was extremely low (about 0.1% the activity of the other enzymes) throughout all areas studied and did not differ significantly between areas or diagnostic groups.