Hypoosmotic exposure (205 mosmol/l) of rat liver macrophages (Kupffer cells) for 12 h stimulated phagocytosis of latex particles by about 20%, whereas hyperosmotic exposure (405 mosmol/l) resulted in 30-40% inhibition. Inhibition of phagocytosis by hyperosmolarity was fully prevented in the presence of betaine, which acts as an osmolyte in liver macrophages. When hyperosmotically exposed Kupffer cells were preloaded with betaine, induction of phagocytosis by addition of latex particles led to the stimulation of betaine efflux from the cells. Stimulation of phagocytosis also inhibited the hyperosmolarity-induced cumulative uptake of betaine into Kupffer cells, but did not prevent the hyperosmolarity-induced increase in BGT1-mRNA levels. Whereas these findings suggest an involvement of cell volume and betaine in the regulation of phagocytosis in Kupffer cells, betaine transport was not affected upon induction of phagocytosis in RAW 264.7 mouse macrophages. The findings are compatible with a role of betaine in maintaining cell volume homeostasis during phagocytosis in Kupffer cells, but not in RAW 264.7 mouse macrophages. This may be relevant for the maintenance of liver hemodynamics, since volume changes of liver macrophages following ingestion of phagocytozable material might otherwise impair sinusoidal perfusion.