Abstract : Humans and most other organisms manifest circadian (daily) rhythms that are controlled by an endogenous biochemical oscillator. These 'biological clocks' are important to human physiology. For example, psychiatric and medical studies have shown that circadian rhythmicity is involved in some forms of depressive illness, 'jet lag', drug tolerance/efficacy, memory, and insomnia. Therefore, understanding the biochemical mechanism of circadian clocks may lead to procedures which will be useful in the diagnosis and treatment of disorders that are relevant to sleep, mental health, and pharmacology. Although recent breakthroughs in the field of circadian rhythms have identified a number of proteins that appear to act as clock components, we have only just begun to understand how these components interact functionally with themselves and the environment to generate a highly precise 24 hour oscillation that is temperature compensated and entrained to the daily cycle. We will test hypotheses concerning the significance of rhythmic clock protein abundance by using new methods to introduce proteins directly into cells by peptide-mediated transduction across cell membranes. These studies will yield results of theoretical importance, but also have the potential for designing treatments for jet lag, insomnia, and other clock-related disorders.