Plasmalemmal caveolae were first identified as an endocytic compartment in endothelial cells, where they appear to move molecules across the cell by transcytosis. More recently, they have been found to be sites where small molecules are concentrated and internalized by a process called potocytosis. A growing body of biochemical and morphological evidence indicates that a variety of molecules known to function directly or indirectly in signal transduction are enriched in caveolae. This raises the possibility that a third function for caveolae is to process hormonal and mechanical signals for the cell. Insights gained from studying potocytosis suggest several different ways that this membrane specialization might function to integrate incoming and outgoing cellular messages.