It has been previously hypothesized that hyperactivity may be one childhood form of manic-depressive disorder. The authors contend that it is unlikely that hyperactivity in general is a childhood form of manic-depressive disorder. However, included in those considered to be hyperactive may be youngsters exhibiting an embryonic form of mania. A case of a 5-year-old boy, originally thought to be hyperactive, is presented as evidence for the existence of an embryonic stage of mania. The clinical and research necessity for differentiating between hyperactivity and embryonic mania is stressed.