Kidney epithelial cell contacts of Helisoma have been studied under different osmotic conditions (hyposmotic, isosmotic, control). In hyposmotic medium 20% of the observed zonulae adherentes appear opened, and septate junctions appear short with few septa or long with many septa showing irregular alignment and periodicity; intercellular spaces are wide. Snails from isosmotic medium show long septate junctions with compact regular septal periodicity and greatly reduced intercellular spaces. The length of septate junctions and the number of septa appear to be related to intercellular space width. Lanthanum-impregnated septa appear pleated and punctate. In freeze-fracture replicas septate junctions from hyposmotic medium have fewer rows or irregular sinuous rows of junctional particles. Osmotically induced changes in the junctional complexes are reversible. In vitro treatment with crude brain extract dramatically increases intercellular space width with significant changes in the junctional complexes. It is proposed that the septate junctions are involved in osmoregulation and a brain factor may regulate them. Gap junctions are found in various places in the kidney cell membranes; they may provide additional structural integrity in the epithelium. Rhombic particle arrays and desmosome-like particles have been observed in freeze-fracture replicas but their identity is unknown and no clear correlation for these particles in thin sections has been possible.