In mammals, the biological clock that drives circadian rhythms in physiology and behavior is located in the anterior hypothalamus in a region known as the suprachiasmatic nuclei (SCN). Although the rhythm generation mechanism is an intrinsic property of the SCN, the phase of the SCN circadian clock is regulated by certain environmental and homeostatic influences. For example, SCN-driven rhythms are normally synchronized, or “entrained”, to the environmental light-dark cycle. This occurs as a consequence of daily, light induced adjustments of the phase of the circadian clock. Work in my laboratory is currently focused on the neurochemical mechanisms through which environmental light, and other phase altering stimuli, “reset” the circadian clock in the rodent brain. Toward this end, we employ a variety of experimental approaches, including brain slice neurophysiology, optical and electrical recording of neuronal activity in primary cultures of SCN neurons, molecular biological approaches, intracerebral microdialysis and behavioral neuropharmacology. Of particular interest are: (1) the neurochemical events responsible for the regulation of circadian phase by light; (2) the role of serotonin in the regulation of circadian phase; and (3) circadian clock control of gene expression in the SCN. It is anticipated that a detailed understanding of the neurochemical mechanisms responsible for resetting the circadian clock will support the development of pharmacological strategies to control circadian phase in humans.