The zonula occludens (Z.O.) has been implicated as a major pathway for passive fluid movement from lateral intercellular spaces to lumen in response to an expansion of blood or fluid volume. The present study examines to Z.O. of adult rat nephrons in control and Ringer-Locke or while blood infused animals to determine whether varying the conditions of passive fluid movement would influence the structure of the Z.O. Rats were infused over 20 minutes with Ringer-Locke (7% body weight) or while blood (2.3% body weight). In the controls, 92%-94% of all Z.O. observed in proximal tubules and 100% of Z.O. from distal tubules and collecting ducts had fused outer leaflets. The approximate linear extent of the Z.O. was 140 A for proximal tubules, 2,500 A for distal tubules and 3,000 A for collecting ducts. There was no significant difference in any of these values following the infusion with either Ringer-Locke or whole blood. We conclude that expansion of blood or fluid volume causes no detectable alteration in the fine structure of Z.O. under the conditions used in the present study.