This paper presents the first of a three-part report on behavioral and biological aspects of the "switch process" in a group of manicdepressive patients. It documents a regular temporal pattern of change in thought and mood observed during ten spontaneous episodes of switches from depression into mania and seven episodes of switches out of mania into depression. Patients were described in detail and their behavior rated every eight hours on a longitudinal basis throughout these periods of change. The depressive period prior to the switch into mania was characterized by moderate depression, seclusiveness, nonverbalization, and dozing. Each of the ten episodes of mania was preceded by a normal or transitional period which was manifested by sudden appearance of spontaneous speech and motor activity and normalization of mood. Total sleep time was markedly reduced on the day prior to the switch into mania and remained low during the manic period.