The effect of long-term lithium carbonate treatment on thyroid function was investigated in 40 clinically euthyroid patients with Manic Depressive Psychosis. Before lithium treatment, all patients showed normal levels (T(4)) of serum total thyroxine, free thyroxine index and thyrotropin (TSH). After lithium carbonate treatment for a mean period of 24 months, these patients did not show any significant change in the mean values of serum total T(4), free thyroxine index and TSH. None showed clinical symptoms of goitre and hypothyroidism after long-term lithium treatment. It is concluded that lithium therapy in euthyroid psychiatric patients has no adverse effect on thyroid function and en the pituitary secretion of TSH.