We have studied the in vitro exposure to various oxysterols on isolated rat platelets. The oxysterols (1-20 uM) were incubated either dissolved in methanol or as albumin-bound complexes. Aggregation (analyzed by turbidimetry) was measured after stimulation by thrombin or ADP. Serotonin secretion (analyzed by voltammetry) was measured after thombin stimulation. We found that platelet aggregation and serotonin secretion could be either not significantly affected (7B-hydroxy cholesterol) or potentiated (22S-hydroxy cholesterol, 3,5,6-hydroxy cholestan triol) or inhibited (25-hydroxy- and 7-oxo cholesterol), after in vitro incubation with different oxysterols. Our data indicate that a modulation of the platelet behavior occurs after in vitro incubation with different oxysterols, some derivatives acting as inhibitors and others as potentiators. These results provide new interesting information regarding the role of these sterols in cell membrane structure and function in relation to pathology.