The direct effects of mechanical compression on axoplasm and fast axoplasmic transport were studied by video-enhanced differential interference microscopy. Single axons, isolated from the squid, were compressed with 0.5, 5, 20, or 100 gram (g) weights placed over a 1 millimeter (mm) length of axon. Brief compressions (10 seconds) at low pressures (0.5 g/mm) momentarily deformed the axon, but the axoplasm and axon returned to their normal shape and position after the pressure was removed, and no residual changes in axoplasmic structures, fast axoplasmic transport or membrane function were seen. Compressing the axon with 5-20 g/mm, however, broke the axoplasm at the site of the crush and squeezed the axoplasm out from under the compression site. Though the axoplasm usually returned to the crush site after the weight was removed and organelles continued to move in the axoplasm under the crush, the organelles failed to cross a dense line that marked the site of the rejoined axoplasm, instead they accumulated over time at the crush site. This results suggests that the blockage of fast transport at moderate compressions was due to a mechanical breakage of the axoplasm at the compression site. The plasma membrane was apparently not transected after moderate compressions (5-20 g/mm) since the resting membrane potential returned to nearly control levels after the weight was removed. Compressions with 100 g/mm, however, did break the plasma membrane as evidenced by the rapid and irreversible loss of the action potential and resting potential and the ion-dependent liquefaction of axoplasm and loss of all organelle transport at the 100 g/mm compression site. Thus, small mechanical pressure elastically deformed the axoplasm, moderate pressures mechanically broke the axoplasm, and high pressures broke the axoplasm and the plasma membrane.