The nature of the delayed blood-brain barrier (BBB) opening that occurs in rats subjected to forebrain ischemia by the technique of two-vessel (carotid) occlusion plus hypovolemic hypotension (2VO ischemia) was probed by examining the simultaneous, trans-barrier movement of two hydrophilic, normally poorly permeative solutes of markedly different molecular size: sucrose (MW=342) and inulin (MW≈5000). Pentobarbital-anesthetized, male, Sprague–Dawley rats (342–374 g) were subjected to 10 min of 2VO ischemia (tympanic temperature, 37.5–38.0°C); 6 h later they were reanesthetized and, along with non-ischemic controls, injected i.v. with [ 14C]sucrose and [ 3H]inulin. Transfer constants ( K is) for blood-to-brain movement of the tracers and V is (apparent initial volumes of tracer distribution) were determined for six brain regions by the multiple-time, graphical method (tracer circulation times from 3 to 30 min). V is differed little or insignificantly between the two tracers, or between control and post-ischemic rats; the values did not suggest appreciable endothelial binding of either tracer that might lead to its uptake by adsorptive-phase endocytosis. In the controls, regional K is±S.E.M. (nl g −1 s −1) for inulin ranged from 0.18±0.04 to 0.31±0.09 and were significantly lower ( P<0.01) than K is for sucrose (1.53±0.16–1.91±0.29). The K i ratio (sucrose/inulin) across brain regions (mean, 6.6; S.E.M., 0.6) was much lower than would be expected according to the concept that movement of most organic non-electrolytes across the intact BBB occurs by dissolution in and diffusion through endothelial cell plasma membranes, at a rate proportional to the lipid solubility and diffusivity of the solute. This finding is interpreted as indicating that a portion of the transfer of sucrose and inulin occurred by a mechanism other than dissolution-diffusion (e.g., via pores or vesicles). In the post-ischemic rats, K is for both tracers were elevated significantly ( P<0.01) in parietal cortex, striatum, hippocampus, and midbrain. The post-ischemic increases ( Δ K is) in these regions were greater for sucrose (1.90–3.31 nl g −1 s −1) than for inulin (0.80–1.33). Across brain regions the ratio between sucrose Δ K i and inulin Δ K i averaged 2.9 (S.E.M., 0.2), a value significantly greater than the ratio of 1 that would be expected were the BBB opening due to an enhancement of micropinocytosis and vesicular transport. The correspondence of the mean Δ K i ratio with the ratio of the free diffusion coefficients of the tracers ( D f, suc/ D f, inu=2.9; water, 38°C) suggests that the delayed opening of the BBB following 2VO ischemia involves the formation of trans- or paracellular, aqueous pores or channels.