PREDICTABLE PATTERNS OF VARIATION occur in almost all biological phenomena at all levels of organization, from molecules in single cells to vital signs and social behavior of human beings. Many of these rhythms involve a variety of internal timing processes that are only partly under environmental control. Some knowledge of these rhythms is essential in understanding life processes, is of central importance in designing and interpreting many kinds of biological research, and has important implications for defining health and detecting and treating disease. Yet the rhythmic variations in biological phenomena are almost entirely ignored in biology textbooks. Indeed, they are often denied in an effort to teach students the idea of homeostasis. In this brief article, we will illustrate the short shrift that rhythmic variations get in five major high school texts, sketch some of the basic facts and principles of biological rhythms, and suggest how the teaching of homeostasis can be revised to accomodate the rhythmic facts. Unhappily, it is necessary first to distinguish sharply between scientific knowledge of biological rhythms and the popular fad of universal, invariant birth-date biorhythms. The silly origins and character of that numerological scheme have been nicely described in the Mathematical Games section of Scientific American (Gardner 1966). The scientific research on real rhythms-which are somewhat variable, different for different people, and not predictable from birthdate-is described in thousands of articles in science and medical journals. [The NIMH monograph Biological Rhythms in Psychiatry and Medicine (Luce 1970) includes 70 pages of bibliography, exclusive of much research since its publication. Some recent developments in theory and applications are illustrated by Franz Halberg (1970), director of the Chronobiology Laboratories at the University of Minnesota.] Because purveyors of the biorhythm fad use similar language, run computers, and deliberately mix the numerology with real research, most people are confused about what is fact and what is fiction. More on the fad appears in the box on the next page.