- Background: %Oral myo -inositol (12–18 g/day) has shown beneficial effect in placebo-controlled studies of major depression, panic disorder, and obsessive compulsive disorder, and preliminary data suggest it also may be effective in bipolar depression. Evidence linking antidepressant activity to membrane phospholipid alterations suggested the examination of acute and chronic myo -inositol effects on rat brain membrane phospholipid metabolism. Methods: With both 31 P nuclear magnetic resonance (NMR) and quantitative high-performance thin-layer chromatography (HPTLC; hydrolysis) methods, rat brain phospholipid levels were measured after acute ( n = 20, each group) and chronic myo -inositol administration ( n = 10, each group). With 31 P NMR, we measured myo -inositol rat brain levels after acute and chronic myo -inositol administration. Results: Brain myo -inositol increased by 17% after acute myo -inositol administration and by 5% after chronic administration, as compared with the control groups. Chronic myo -inositol administration increased brain phosphatidylethanolamine (PtdEtn) plasmalogen by 10% and decreased brain PtdEtn by 5%, thus increasing the ratio PtdEtn plasmalogen (PtdEtn-Plas)/PtdEtn by 15%. Phosphatidylethanolamine plasmalogen levels quantified by 31 P NMR and HPTLC were highly correlated. The validity and reliability of the 31 P NMR method for phospholipid analysis were demonstrated with phospholipid standards. Conclusions: The observed alteration in the PtdEtn-Plas/PtdEtn ratio could provide insights into the therapeutic effect of myo -inositol in affective disorders.