In addition to its incorporation into proteins, glycine functions as both a regulator of one-carbon metabolism and as an inhibitory neurotransmitter. Clinical recognition of the hyperglycinemias and reported elevations of erythrocyte glycine concentrations in patients with bipolar disorders have implicated this amino acid in the etiopathology of some neuropsychiatric disorders. Moreover, chronic lithium administration, an almost specific intervention for the treatment and prophylaxis of bipolar disorders, has been shown to induce elevations in brain and erythrocyte glycine levels. In view of glycine's complex metabolic interrelationships and neurotransmitter function, additional research exploring its possible role in either affective illness or lithium's action is indicated.