Mesenchymal cells, termed cushion tissue (CT) cells, are the principal cellular elements in atrioventricular (AV) endocardial cushions, the latter constituting AV septal and valvular primordia in the embryonic heart. Atrioventricular canals of 2½ day chick embryo hearts were explanted onto collagen gel lattices, wherein cushion endothelial cells acquired the characteristics of CT cells and invaded the gel. The endocytic activity of in situ CT cells was compared to that of gel-cultured CT cells by exposing appropriate preparations to horseradish peroxidase (HRP), cationized ferritin (CF) or to lysozyme. Stimulation by these exogenous proteins resulted in phagocytosis. In addition, all three markers were associated with coated pits, smooth surfaced vesicles (45–60 nm), C- or cup-shaped structures and other lysosomal elements, but were excluded from Golgi cisternae and their entire vesicle population. In CT cells, HRP does not act solely as a “content” marker that reflects fluid-phase uptake. Instead it appears to follow adsorptive endocytic pathways. The endccytic behavior of in vivo and in vitro cushion tissue cells appears to be the same. Endogenous endocytic activity may reflect in part membrane retieval, which counterbalances the extensive exocytosis observed in these cells.