Cholinergic processes were measured in plasma and red blood cells (RBC) obtained from patients with mania, depression, and schizophrenia. RBC choline levels were elevated in manic patients, and lithium treatment led to a further increase. RBC choline transport was below normal in manic patients, and lithium treatment further reduced choline transport in addition to reducing the apparent affinity for choline. RBC acetylcholinesterase was low in depressed and schizophrenic patients, but not in manic patients, whereas plasma acetylcholinesterase was reduced only in depressed patients. Plasma nonspecific cholinesterase was below normal in all groups of patients. These results indicate unique patterns of differences from controls in the cholinergic system in blood samples from patients with different psychiatric illnesses. © 1985.