A common event that occurs during apoptosis is a loss of cell volume, but little information is available on its role in the cell death process. Lymphocytes undergo apoptosis in response to glucocorticoids and exhibit cell shrinkage, nuclear condensation, internucleosomal DNA fragmentation, and apoptotic body formation. Interestingly, only cells that exhibit a loss in cell volume degrade their DNA. To determine if physical shrinkage was sufficient to initiate apoptosis, S49 Neo lymphocytes were cultured in hypertonic medium. The normal osmolarity (approximately 300 mosM) of tissue culture medium was increased to either 550 or 800 mosM, using impermeant sugars such as mannitol and sucrose or NaCl. These hypertonic conditions led to a rapid killing of S49 Neo cells. Evaluation of the mode of cell death revealed that these hypertonic conditions resulted in apoptosis. Unlike glucocorticoid-induced cell death, hypertonically induced apoptosis did not require protein synthesis. When S49 Neo cells were cultured under hypotonic conditions, the cells swelled but apoptosis did not occur. Analysis of several cell types revealed that all lymphoid cells examined (S49 Neo, CEM-C7, primary thymocytes) undergo apoptosis in response to hypertonic conditions, whereas several other cell types (L cells, COS, HeLa, GH3) did not. Although these nonlymphoid cells showed a similar initial reduction in cell volume in response to hypertonic conditions, they subsequently maintained volume or regulated back to a near normal cell volume. These data indicate that thymic lymphoid cells have the machinery in place for rapid induction of apoptosis in response to physical shrinkage, whereas other cell types resist shrinkage-induced apoptosis by the activation of cell volume regulatory mechanisms.