A quantitative study has been made of the retrograde neuronal changes that occlr in the large neurons of Clarke's column in the L3 spinal segment, 100 days after dorsolateral column lesions at the medullary, C5, T1, T8, and L1 levels in a series of 10-week-old kittens. Cell counts, and nuclear and perikaryal areal measurements, have been made in thionin stained material. The results indicate that following L1 level lesions, there is about a 30% loss of Clarke neurons and the remaining neurons are reduced in size to about 75% of their normal cross-sectional perikaryal area (as judged from the cells on the opposite side) and about 65% of their normal nuclear area. Lesions at the T8 and T1 level both cause about a 20% Clarke cell loss; but after the T8 lesions, there is about a 65% decrease in perikaryal area and a 55% decrease in nuclear area while the T1 lesions cause about a 55% decrease in perikaryal area and a 45% reduction in nuclear area. Lesions at the C5 level result in about a 15% cell loss, a 50% decrease in perikaryal area, and a 40% decrease in nuclear area. Medullary lesions produce about a 5% cell loss, a 50% decrease in perikaryal area, and a 40% decrease in nuclear area. In unoperated control preparations, the left-right differences in the number of cells are less than 2% and the areal measurements are within 5% of each other. The results of this study suggest that the severity of the retrograde neuronal reaction in Clarke's column is related to the proximity of the lesion to the cell body, but that this relationship is non-linear. Several possible interpretations of these results are discussed.