1. When pigs were fed diets deficient in pyridoxine or pantothenic acid, ataxia developed and lesions were found in the sensory neuron. 2. The morphological pattern of the lesions differed in the early stages of the two deficient states. Degeneration of the peripheral process of the sensory neuron was the initial and most prominent feature in pyridoxine deficiency. Chromatolysis was the first evidence of damage to the afferent neuron in pantothenic acid-deficient animals. 3. The possible significance of this evidence of damage to different portions of the sensory neuron is discussed.