The data reported in this paper support the hypothesis that the neurotransmitter catecholamines (dopamine and/or norepinephrine) are functionally increased prior to the switch from depression into mania. Urinary norepinephrine was significantly elevated on the day prior to and during the manic episodes. Overall sleep and rapid eye movement were both decreased prior to and during the switch into mania in a few patients. Our pharmacological data is also supportive of this hypothesis. Brief administration of tricyclic medication has been associated with rapid switches from depression into mania. Large doses of levodopa (which increases brain dopamine) were regularly associated with brief hypomanic episodes in patients with past histories of mania. On the other hand, αmethyl-ρtyrosine, which inhibits the synthesis of norepinephrine and dopamine, was associated with a transitory switch out of mania in a few patients.