Chronic administration of tricyclic antidepressants is common; folic acid depletion is a potential consequence adversely affecting the mental state. In a pilot study prior to research in the community, serum and red cell folate and serum vitamin B (12) levels were measured in the following elderly psychiatric inpatients: 14 controls (patients not receiving any drugs with known antifolate activity), 11 receiving tricyclic antidepressants, 13 receiving antipsychotics (phenothiazines) and four receiving an anticonvulsant (carbamazepine). Patients on prolonged treatment with carbamazepine or phenothiazine drugs had lower concentrations of folate in serum and erythrocytes compared with controls; the decrease was statistically significant for the effect of phenothiazines on serum folate levels. Tricyclic antidepressants, which are in widespread use in the community, did not cause folate depletion during the first two years of treatment.