Ultrastructural changes in the cytoplasm of transected dendrites have been studied with serial section electron microscopy 10 to 12 min after the lesion. Dendrites from large multipolar neurons in mouse spinal monolayer cultures were selected 4 weeks after seeding and cut with a pulsed UV laser microbeam. The data reveal cytoplasmic damage gradients which decrease in severity with distance from the lesion. The damage is expressed primarily in cytoskeletal disruption, the swelling and vacuolation of mitochondria (MC), and the swelling and vesiculation of the endoplasmic reticulum (ER). For a 3-4 microns diameter dendrite lesioned 150 microns from the soma and fixed 10 min after transection, the following results can be reported. The number of microtubules (MT) decreases almost linearly with a slope of 40 MT/10 microns until MT completely disappear 20 microns from the lesion. Neurofilaments (NF) decrease at approximately 50 NF/10 microns, with 20% of these structures still intact at or near the lesion. MC show both gradual and explosive swelling in the damaged process. The onset of these two phases of MC swelling are correlated with the initial and total loss of MT respectively. MT and NF densities in cut dendrites are drastically different from those in control processes and suggest a possible retrograde redistribution of these organelles. Damage from external shockwaves that cause process distortion but not transection results in a less severe but nevertheless similar loss of MT and NF.