This review of recent research suggests the existence of an unstable state of central nervous system hyperexcitability in depression, and possibly in mania, with an associated but disorganized intrinsic hyperactivity. The authors propose this concept as a possible description of the prevailing neurophysiologic state of the individual with affective disorder and as complementary to other biological hypotheses currently under investigation. They conclude that the concept warrants further study, noting the need for replication and extension of many of the studies reviewed.Abstract Teaser