A basic conservation of cell migration guidance mechanisms in the nervous and immune systems was proposed when Slit, known for its role in axon guidance, was found to inhibit chemokine-induced leukocyte chemotaxis in vitro. These studies examined the role of Slit2 in modulating inflammation in vivo. In a rat model of glomerulonephritis, endogenous glomerular Slit2 expression fell after disease induction, and its inhibition during the early disease period accelerated inflammation. Ex vivo glomerular leukocytes showed decreased chemokine and chemoattractant-induced chemotaxis in response to Slit2, suggesting an anti-inflammatory role for glomerular Slit2. In contrast to the effect of inhibition, glomerulonephritis was ameliorated by systemic Slit2 administration. Slit2 treatment improved disease histologically and also improved renal function when given early in the disease course. Leukocytes harvested from rats receiving Slit2 showed decreased monocyte chemoattractant protein-1 (MCP)-1-mediated migration, consistent with a peripheral Slit2 effect. In keeping with this functional alteration, Slit2-mediated inhibition of RAW264.7 cell chemotaxis was associated with decreased levels of active cdc42 and Rac1, implicating GTPases in leukocyte Slit2 signaling. These findings suggest a role for endogenous Slit2 in the inhibition of chemoattractant- mediated signals, demonstrate a potentially important anti-inflammatory effect for Slit2 in vivo, and provide further evidence for conserved mechanisms guiding the process of migration in distinct cell types.