OBJECTIVE Lysophosphatidic acid (LPA) increases permeability of cerebral endothelium in culture, but it has been suggested that histamine release is required in vivo. METHODS Cerebral venular permeability was measured by using the single-vessel micro-occlusion technique, and fura-2 ratios were used to track changes in endothelial [Ca(2+)]. RESULTS Topical acute LPA application dose-dependently increased permeability (log EC(50)-9.4; similar to the K(d) of the LPA1 receptor). The calcium response to LPA was similar to histamine, but the permeability response was unaffected by H(2)-histamine receptor antagonism, and was blocked by Ki16425, a LPA1 receptor antagonist. The permeability response was blocked by nitric oxide synthase and free radical scavenging, which were carried out together, but not separately. Intravascular LPA bolus injection increased permeability. Whole serum albumin, or plasma albumin co-applied with LPA, increased permeability, but less potently than LPA itself (log EC(50) 5.1 and 6.1, respectively). Tachyphylaxis of the LPA1 receptor was demonstrated by LPA application for 10 minutes, which resulted in suppression of the response to subsequent applications for the following 15 minutes. CONCLUSIONS Lysophosphatidic acid increases cerebrovascular permeability by acting directly on the endothelium and utilizes both nitric oxide and free radical signaling pathways.