This research seeks to define the role and mechanisms whereby serotonin (5-hydroxytryptamine, 5-HT) affects, or participates in, the control of neuroendocrine systems. Primary attention selected the mechanisms of the hypothalamo-hypophyseal axis for the chief tissue sites of this research. Within these regions our interest will continue to center on the suprachiasmatic nuclear region of the hypothalamus and the controls impinging on it through serotonergic nerve terminals and other neuroendocrine components, including the pineal gland. It is believed that improved knowledge concerning these mechanisms and their controls will contribute to new approaches and interventions for optimizing human neuroendocrine and reproductive functions in development, health and disease.