A description of Periodic Catatonia is offered stressing its relatively well-defined nature and the unique opportunity for controlled investigation that it presents. A case is described and discussion is offered of findings, hypotheses and therapy described in the literature. Discussion is particularly devoted to the investigation of the role of the thyroid and treatment with thyroid substances in this disorder. Spontaneous seizures were observed in the case described. Comment is made upon these (ictal) phenomena and E.E.G. changes that may occur in cases of Periodic Catatonia and may bear a fairly constant relationship to other phasic alterations (possibly of nitrogen balance) in this illness.