Two newborn and three young adult monkeys ( Macaca mulatta) were hemidecorticated by surgical aspiration and allowed to survive postoperatively for 15 to 31 days. The brains were perfused, sectioned transversely and stained with thionine. Resulting retrograde cell degeneration in the diencephalon and various other structures was examined microscopically and a comparison made between the newborn and adult animals. In the infant monkeys the following thalamic nuclei showed heavy cell loss: anterior dorsalis; anterior ventralis; anterior medialis (lateral two-thirds); ventralis lateralis; ventralis posterior lateralis; ventralis posterior inferior; lateralis dorsalis; lateralis posterior; reticularis; medialis dorsalis, partes parvocellularis and multiformis; pulvinaris; geniculatus lateralis dorsalis; geniculatus medialis, pars parvocellularis; and suprageniculatus. Nuclei showing partial cell loss were: anterior medialis (medial one-third); medialis dorsalis, pars densocellularis; reuniens; centralis latocellularis; alaris; centralis lateralis; paracentralis; limitans; and ventralis posterior medialis. Unaffected or only slightly affected nuclei included: ventralis anterior; medialis dorsalis, pars magnocellularis; geniculatus medialis, pars magnocellularis; centrum medianum; parafascicularis; centralis superior lateralis; and assorted midline nuclei. Thalamic degeneration in the adults was similar to the infants with the following exceptions which showed more severe cell loss in the adults: ventralis anterior; medialis dorsalis, pars magnocellularis; geniculatus medialis, pars magnocellularis; centralis lateralis; and paracentralis. Additional degeneration was present in three extrathalamic structures: the medial mamillary nucleus, which showed severe cell loss in the infants and partial cell loss in the adults; the caudal half of the nucleus of the diagonal band of Broca, which had virtually disappeared; and the ipsilateral pontine nuclei which were partially affected in the infants. No differences in extent of degeneration could be ascribed to variation in postoperative survival times.