Acute photochemical demyelination accompanied by minor axonal degeneration was produced in rat sciatic nerve after topical application of the photosensitive dye Rose Bengal and focal illumination with cold light. Animals were sacrificed at different time periods after challenge and the exposed nerves prepared for light microscopic and ultrastructural evaluation. Important structural changes were already observed at 4-6 h. These included endoneurial swelling, diapedesis of neutrophils and monocytes, vacuolization and vesicularization of Schwann cell cytoplasm, lamellar separation of myelin sheaths, disintegration of axonal microtubules, and accumulation of vesicular material and mitochondria in the axoplasm. Disrupted myelin fragments were phagocytosed by macrophages which penetrated Schwann tubes at Day 3. Schwann cells proliferated and started to enwrap denuded segments of the axon. They were surrounded by redundant basal lamina, thrown into deep folds. Axons remained partly hypertrophic and contained many neurofilaments. A minority showed signs of degeneration. At Days 5-7 denudation was almost complete in the light-exposed nerve area but also in small distal nerve fascicles. After 1 month, axons in the illuminated area and distal to it were completely remyelinated although they had thinner sheaths. Exposure to increased light intensity resulted in deeper lesions and more extended anterograde damage, which also recovered within 1 month. All animals showed rapid functional deterioration which correlated with the severity and extent of structural damage. Recovery was slow and also depended on the degree of histologic damage. Neither control nerves nor sham-exposed nerves revealed signs of structural or functional changes.(ABSTRACT TRUNCATED AT 250 WORDS)