Sphingosine 1–phosphate (S1P) is a key endogenous regulator of the response to lung injury, maintaining endothelial barrier integrity through interaction with one of its receptors, S1P₁. The short-term administration of S1P or S1P₁ receptor agonists enhances endothelial monolayer barrier function in vitro, and attenuates injury-induced vascular leak in the lung and other organ systems in vivo. Although S1P₁ agonists bind to and activate S1P₁, several of these agents also induce receptor internalization and degradation, and may therefore act as functional antagonists of S1P₁ after extended exposure. Here we report on the effects of prolonged exposure to these agents in bleomycin-induced lung injury. We demonstrate that repeated administration of S1P₁ agonists dramatically worsened lung injury after bleomycin challenge, as manifested by increased vascular leak and mortality. Consistent with these results, prolonged exposure to S1P₁ agonists in vitro eliminated the ability of endothelial cell monolayers to respond appropriately to the barrier-protective effects of S1P, indicating a loss of normal S1P–S1P₁ signaling. As bleomycin-induced lung injury progressed, continued exposure to S1P₁ agonists also resulted in increased pulmonary fibrosis. These data indicate that S1P₁ agonists can act as functional antagonists of S1P₁ on endothelial cells in vivo, which should be considered in developing these agents as therapies for vascular leak syndromes. Our findings also support the hypothesis that vascular leak is an important component of the fibrogenic response to lung injury, and suggest that targeting the S1P–S1P₁ pathway may also be an effective therapeutic strategy for fibrotic lung diseases.