Mechanical forces are ubiquitous modulators of cell activity but little is known about the mechanical stresses in the cell. Genetically encoded FRET-based force sensors now allow the measurement of local stress in specific host proteins in vivo in real time. For a minimally invasive probe, we designed one with a mechanical compliance matching that of many common cytoskeleton proteins. sstFRET is a cassette composed of Venus and Cerulean linked by a spectrin repeat. The stress sensitivity of the probe was measured in solution using DNA springs to push the donor and acceptor apart with 5-7 pN and this produced large changes in FRET. To measure cytoskeletal stress in vivo we inserted sstFRET into α-actinin and expressed it in HEK and BAEC cells. Time-lapse imaging showed the presence of stress gradients in time and space, often uncorrelated with obvious changes in cell shape. The gradients could be rapidly relaxed by thrombin-induced cell contraction associated with inhibition of myosin II. The tension in actinin fluctuated rapidly (scale of seconds) illustrating a cytoskeleton in dynamic equilibrium. Stress in the cytoskeleton can be driven by macroscopic stresses applied to the cell. Using sstFRET as a tool to measure internal stress, we tested the prediction that osmotic pressure increases cytoskeletal stress. As predicted, hypotonic swelling increased the tension in actinin, confirming the model derived from AFM. Anisotonic stress also produced a novel transient (~2 minutes) decrease in stress upon exposure to a hypotonic challenge, matched by a transient increase with hypertonic stress. This suggests that, at rest, the stress axis of actinin is not parallel to the stress axis of actin and that swelling can reorient actinin to lie more parallel where it can absorb a larger fraction of the total stress. Protein stress sensors are opening new perspectives in cell biology.