Rapid cycling includes some rare manifestations which appear to have a highly biological background, probably coupled to the circadian rhythm and Zeitgeber. Increased cortical norepinephrine (noradrenaline) and decreased 5-hydroxytryptamine and dopamine turnover has been described in bipolar patients. Concerning an impact of the serotonergic system, a very high and, compared to bipolar disorder in general, increased comorbidity has been described between mixed states, obsessive-compulsive disorder, and anxiety disorders, which are generally considered as serotonergic disorders. Besides acting on different neurotransmitters, antiepileptic drugs mainly target transmembranous ion fluxes. Mobilization of calcium is a key event in presynaptic and postsynaptic signalling and also in lasting neuronal changes, as long-term potentiation. The effects of antiepileptic drugs that are efficacious in mixed states and rapid cycling also include a variety of intracellular action targeting the protein kinase activity, the inositol phosphate metabolism, and finally the expression of early genes and cytoprotective proteins.