The purpose of the NIMH PDSP is to provide screening of novel synthetic compounds and natural products for potential use as research tools or probes for basic and clinical research, as therapeutic agents for mental disorders, and as positron emission tomography (PET), single photon emission computed tomography (SPECT), and functional magnetic resonance imaging (fMRI) ligands for functional brain imaging. This program is primarily intended as a screen for compounds that have previously been shown to possess pharmacological, biochemical, or behavioral activities relevant to NIMH and not for the purpose of large-scale, random screening of natural products or combinatorial libraries (e.g., libraries containing >1000 compounds). However, screening maybe conducted on small libraries of synthetic or natural product compounds of unknown activity and on hit or lead compounds generated through NIH-funded high throughput screening (HTS) or chemistry programs.