A dual pathway model is proposed for transport across the tight junction (TJ) strands in rat proximal tubule epithelium: large slit breaks formed by widely dispersed discontinuities in the TJ complex, and numerous small circular pores, with spacing similar to that of claudin-2 within the TJ strand. This dual pathway model is developed as a component of the proximal tubule epithelium model of Weinstein (Am. J. Physiol., 247:F848, 1984) in order to provide an ultrastructural view of the movement of water, ions, and nonelectrolyte solutes across proximal tubule. The predictions of the compartment model for the TJ reflection coefficient and water permeability, and the measured epithelial NaCl and sucrose permeability provide a set of constraints for the dual pathway model, which in turn yields a continuum of solutions for the small pore radii and spacing and large slit gap height and area. For a small pore spacing of 20.2 nm, comparable to the distance between adjacent particle pairs in apposing TJ strands, one finds that the small pore radius is 0.668nm and the large slit breaks have a gap height of 19.6nm occupying 0.04% of the total TJ length of proximal tubule. This pore/slit geometry also satisfies the measured permeability for mannitol. The numerous small circular pores account for 91.2 percent of the TJ NaCl permeability, but only 5.0 percent of the TJ water permeability. The infrequent large slit breaks in the TJ account for 95.0 percent of the TJ water permeability, but only 8.7 percent of the TJ NaCl permeability. Small nonelectrolytes, such as sucrose and mannitol (4.6A and 3.6A radius), can pass through both the large slit breaks and the small pores. For sucrose 78.3 percent of the transport is accommodated via the large slit breaks and 21.7 via the small pores. For mannitol, the transport is split nearly evenly between the two pathways, 50.8 and 49.2 percent. In this ultrastructural model, the water permeability of the TJ is 21.2 percent of the entire transepithelial water permeability and thus an order of magnitude greater than the single pore/slit theory results computed by Preisig and Berry (Am. J. Physiol., 249:F124-131, 1985).