Neuropeptides are cell-to-cell signaling molecules that act as neurotransmitters, neuromodulators and hormones that impact a large variety of neuronal processes. The term neuropeptide refers to bioactive peptides made in neuron, stored in vesicles, and released into the extracellular space. While many peptides can be detected in a tissue homogenate, these will include processing intermediates and even protein degradation products. It is of great interest in the field of peptidomics to focus on functional characterization of proteins products such as cell-to-cell signaling peptides that are released from specific neuronal tissues, whether a brain region or specific cell. A variety of analytical techniques have emerged over the years to analyze neuropeptide release, and these methods have enabled scientists to characterize thousands of brain peptides. The focus of this research was on using various sampling approaches coupled to matrix-assisted laser desorption/ionization-mass spectrometry (MALDI MS) to analyze neuropeptide release from rodent brains. Chapter 2 is a general overview of the current state of analytical methods used to characterize neuropeptide release from cells to animals. Chapter 3 highlights two methods demonstrating neuropeptide release in a mouse model of fragile X syndrome. Sampling techniques using synaptoneurosomes and ex vivo brain slices were used to show a neuropeptide release deficit in Fmr1 KO mice. Chapter 4 highlights an approach utilizing in vivo microdialysis coupled to offline MALDI MS. This method was used to characterize extracellular peptide release from the hippocampus of rats in response to saline or morphine injection coupled with a spontaneous alternation task. In particular, fibrinopeptide A, a peptide derived from the fibrinogen α-chain, was significantly upregulated in rats exposed to morphine and spontaneous alternation testing. The functional consequence of fibronopeptide A release is still under investigation. The advancement of such analytical approaches to characterize neuropeptide release from a variety of samples ranging from cells to animals enables new discovery efforts for understanding the physiological and behavioral role of unknown peptides. Advisors/Committee Members: Sweedler, Jonathan V. (advisor), Sweedler, Jonathan V. (Committee Chair), Gillette, Martha U. (committee member), Ceman, Stephanie S. (committee member), Galvez, Roberto (committee member).