The morphological characteristics of neurons containing 200-kDal neurofilaments (NF200 ) were studied in the sensory ganglia of the thoracic and lumbar spinal nerves during the first year of life in rats ( n = 80). Treatment of neonatal rat pups with capsaicin (150 mg/kg) altered the developmental dynamics of NF200 neurons, with a decrease in the numbers and mean cross-sectional areas of these neurons in both ganglia. Segmental differences consisted of large decreases in the number of NF200 neurons in the sensory ganglia of the lumbar spinal nerve in the early and late periods of life.