Tyr-MIF-1, an endogenous neuropeptides with opiate modulating and other CNS effects, can serve as a precursor of MIF-1 in brain mitochondria. Levels of the new tetrapeptide Tyr-MIF-1 are higher in some parts of the brain than in others and can be altered by neuroendocrine manipulations raises the possibility that Tyr-MIF-1 may affect CNS functions regardless of any effects on MSH release.