Different macromolecules were administered i.p. to stimulate formation of protein-rich ascitic fluid in rodents. Stimulatory effect of plant lectins depended on the attachment to cell surface carbohydrates, ConA was used in the majority of experiments. The time course of ConA-induced ascites was divided into an early (up to 4h) and a late phase (from 6h on), with a transitional period between the two. Water and protein accumulation showed parallel time courses: volume of the ascitic fluid peaked at around 3h, and fibrin threads appeared after 6h. Viscosity of the ascitic fluid and its supernatant increased with time, reaching maximal fibrinogen concentration at around 16h. Peritoneal permeability, followed by pleural and pericardial effusions, was elicited only by lectins which form soluble complexes with serum glycoproteins, whereas the effect of serum-precipitating lectins was restricted to the peritoneum. Macromolecules with serial positive charges (e.g. polylysine or polyethyleneimine) enhanced peritoneal permeability by ionic interactions with cell surface molecules. Viscosity of the polycation-induced ascitic fluid did not tend to increase with time and corresponded to the early-phase of the ConA-induced ascites. Polyglutamate, a polyanionic macromolecule, inhibited the effect of polycations, but not that of ConA. The most efficient stimulatory macromolecules appear to induce ascites by non-covalent cross-linking of cell surface glycoproteins or glycosaminoglycans or both. A similar mechanism may operate in the maintenance of basal secretion to prevent eventual desiccation. Non-covalent cross-linking appears to be a common denominator of both basal and enhanced permeability.