Erythrocyte methionine adenosyltransferase (MAT) activity (V<inf>max</inf>) and phosphatidylcholine (PC) levels previously have been found increased in manic patients and decreased in depressive and schizophrenic patients. To evaluate whether these abnormalities were the result of medication effects, erythrocyte MAT activity (V<inf>max</inf>) was assayed for paired samples from 29 schizophrenic, 16 manic, and 12 depressive patients, an erythrocyte PC levels were obtained for paired samples from 13 schizophrenic, seven manic, and seven depressive patients. Patients were medication free for at least 3 weeks. V<inf>max</inf> was significantly increased in schizophrenic and depressive patients (p < 0.01; p < 0.01) and significantly decreased (p < 0.01) in manic patients after 2 weeks of psychotropic medication. Similar trends were found in PC levels. The findings of those onecarbon metabolism tests following medication are generally opposite to those reported to be related to specific disorders and tend toward normalization. Moreover, in vitro preincubation of erythrocytes of three normal subjects with the most commonly used neuroleptics had no consistent effects of MAT V<inf>max</inf> These findings confirm previous studies that showed similarities in onecarbon metabolism of schizophrenic and depressed patients as opposed to manic patients and suggest that medications tend to correct or minimize rather than induce such abnormalities. © 1986 by Williams & Wilkins Co.