1 The content and distribution of 5-hydroxytryptamine (5-HT) between subcellular fractions from rat whole brain (excluding cerebellum) were examined following repeated lithium administration. 2 Lithium chloride (3 mEq/kg body wt. s.c.) administered twice daily for 3 days produced no change in the 5-HT content of primary subcellular fractions (P1-nuclear; P2-crude synaptosomal; SNT-soluble) measured on the 4th day. 3 Similarly, repeated lithium treatment alone did not appear to produce increases in the 5-HT content of either cytoplasmic (S) or vesicular (M2) fractions derived from hypo-osmotically disrupted synaptosomes (P2) when compared to control rats receiving NaCl only. 4 One hour after monoamine oxidase inhibition with tranylcypromine, there was a further selective 25% increase in 5-HT accumulation in the lithium-treated rats over control values in the soluble cytoplasmic fraction (S) and in fractions containing occluded cytoplasm only. This increase did not occur in the synaptic vesicle fraction (M2). This corresponds to an increase in 5-HT turnover rate due to the lithium treatment of approx. 50%. 5 These findings lend additional support to the hypothesis that lithium treatment alters the intraneuronal storage or compartmentation of 5-HT between vesicles and cytoplasm.