Administration to rats of LiCl (3 mEq/kg) subcutaneously twice daily for 3 days followed by monoamine oxidase inhibition with either tranylcypromine (TCP; 20 mg/kg) or pargyline (75 mg/kg) on the fourth day produces a syndrome of hyperactivity indistinguishable from that produced by monoamine oxidase inhibition and l‐tryptophan administration. At least 3 injections of LiCl (3 mEq/kg) are necessary before hyperactivity is seen but one dose of LiCl (10 mEq/kg) 5 h before TCP also caused hyperactivity. The hyperactivity is blocked by prior administration of p‐chlorophenylalanine, a tryptophan hydroxylase inhibitor. LiCl pretreatment does not alter the concentration of l‐tryptophan in the brain. However after monoamine oxidase inhibition the 5‐hydroxytryptamine (5‐HT) accumulation was significantly greater in animals given lithium indicating an increase in 5‐HT synthesis of 70%. This was confirmed by measuring 5‐hydroxyindoleacetic acid accumulation after probenecid (200 mg/kg). The hyperactivity produced by the 5‐HT analogue, 5‐methoxy N,N‐dimethyltryptamine was not potentiated by lithium pretreatment but one injection of LiCl (3 mEq/kg) which did not alter the rate of 5‐HT synthesis, did potentiate the hyperactivity following TCP (20 mg/kg) and l‐tryptophan (50 mg/kg). These results suggest that lithium administration may cause an initial alteration of the 5‐HT available for release at the nerve ending, which is followed after subsequent treatment by an increase in the rate of 5‐HT synthesis. The possible clinical significance of these findings is discussed.