Updated project metadata. Proteins are secreted from various cells to send information to neighboring cells or distant tissues. Because of the highly integrated nature of energy balance systems, there has been a great deal of interest in myokines and adipokines, proteins secreted from muscle and fat tissues, respectively. These have been challenging to study through proteomics because serum is loaded with super-abundant proteins that limit the detection of proteins that have a low, hormone-like abundance. We show here that interstitial fluids (IFs) can be harvested easily from muscle and fat tissues of mice by very low-speed centrifugation and these fluids show a very different protein constellation than that of plasma or tissues. Under several different perturbations in vivo, like exercise or cold, quantitative Mass Spectrometry of these IFs allowed for the identification of many novel myokines and adipokines, including factors both increased and decreased in abundance. Finally, we identify prosaposin, a well-known neurotrophic factor, as a secreted product of both muscle and fat tissues; recombinant prosaposin stimulates a gene program related to thermogenesis in primary fat cells, indicating one potential function for PSAP.