Abstract: This study describes the development of a new technique for long-term measurement of daily 5-hydroxytryptamine (5-HT) and melatonin contents in the pineal gland of freely moving rats. The technique features a number of novel improvements over previous protocols. It allows visualization of the pineal gland for accurate targeting of the guide cannula, which minimizes bleeding; incurs no direct injury to the surrounding brain tissues; and causes no interference with the sympathetic innervation from the superior cervical ganglia. Robust releases of melatonin and indole precursors were continuously monitored quantitatively and reproducibly for more than 2 wk in the same animal. In addition, effects of pharmacological agents on in vivo pineal circadian rhythms can be studied reproducibly over time, and gene expression profiles can be correlated with physiological consequences in single animals. Using these approaches, it is found that β-adrenergic activation leads to decreased release of 5-HT, and that increased cAMP signaling in vivo results in activation of N-acetyltransferase gene induction and melatonin production. These studies will enhance the understanding of signaling pathways that regulate pineal 5-HT and melatonin synthesis and secretion.