Mechanical forces in tissues can produce major changes in the functional behavior of cells. The laser tweezers provide us with the ability to apply and measure forces on cells under conditions where we can document the functional consequences. Our recent observations indicate that membrane- cytoskeleton adhesion controls endocytosis rate and lamellipodial extension rate through an apparent tension in the membrane (reviewed in Sheetz and Dai, 1996). Forces on extracellular adhesive contacts appear to control signaling through the tyrosine kinase/phosphatase pathways (reviewed in Sheetz and Galbraith, 1998). In the first case, the laser tweezers was used to develop tethers from the cell plasma membrane and to measure the force on those tethers as an indicator of the apparent membrane tension. We find that the static tether force is remarkably constant within a given cell type and does not change with tether length. Static tether force is primarily altered by changes in the strength of membrane-cytoskeleton adhesion in non-spherical cells. In turn, changes in the tether force are inversely correlated with changes in the endocytosis rate and the rate of extension of the plasma membrane. We find that the stimulation of secretion in rat basophilic leukemia cells causes a decrease in membrane tension that correlates with the rise in endocytosis rate. In mitosis where there is a block of endocytosis, the tension rises dramatically and a decrease in membrane tension caused by the addition of amphyphilic compounds causes a proportionate increase in endocytosis rate. We hypothesize that many weak interactions between anionic lipids and cytoskeletal proteins serve to hold the membrane and cytoskeleton together in the face of osmotic swelling forces. In the case of adhesive contacts between cells and substrata, we have found that the application of force to integrins by restraining bound fibronectin causes a strengthening of integrin-cytoskeleton attachment (Choquet et al., 1997). The effects are focal in that we observed the changes in the contacts to a bead that experienced a restraining force but adjacent bead-cytoskeleton contacts were not affected. Strengthening of the focal contacts is dependent upon tyrosine kinase/phosphatase pathways in that the inhibition of phosphatases weakens contacts whereas inhibition of kinases appears to strengthen contacts. Substrate rigidity controls cell growth in normal cells whereas transformation is characterized by the ability of cells to grow on soft agar (deformable substratum). We suggest that the development of force results in signals being generated locally and does not involve a long range signaling process as suggested in tensegrity models. References Choquet, D., D. P. Felsenfeld, and M. P. Sheetz. (1997) Extracellular matrix rigidity causes strengthening of integrin-cytoskeletal linkages. Cell 88: 39-48. Galbraith, C. G., and M. P. Sheetz. (1998) Cell migration: Regulation of force on extracellular-matrix-integrin complexes. Trends in Cell Biol. 8: 51-57. Sheetz, M. P., and J. Dai. (1996) Modulation of membrane dynamics and cell motility by membrane tension. Trends Cell Biol. 6: 85-89.