Fine structural changes were observed in the dark scalloped central C1 terminals of type I synaptic glomeruli in spinal cord segments C6-C7 of the rat 3 days after cutting the three main forelimb nerves. Twenty-six per cent of the C1 terminals occurring on the ipsilateral side showed a lighter appearance due to a decrease in the number of synaptic vesicles. The number of synaptic vesicles per unit section area was only 42% of that present in normal C1 terminals on the contralateral side. The number of synaptic contacts of C1 terminals with the profiles surrounding them in each glomerulus was diminished and glial envelopment was increased to 15% of C1 terminal contour. Up to day 12, vesicle and synaptic losses were gradually aggravated and glial apposition was increased, but no obvious signs of glial engulfment were observed. From day 3 to day 12, altered C1 terminals increased in number, while those that appeared normal decreased. The latter had disappeared at day 12 and the altered ones at day 15, and from this stage type I glomeruli were no longer present on the treated side. The lack of electron-dense degenerative bouton changes characteristic of Wallerian degeneration offers an explanation for the lack of or minimal amount of argyrophilic structures which has been found consistently in the substantia gelatinosa during transganglionic degeneration. The gradual decay of the C1 terminals raises the question of their fate. Future studies with the use of a stable marker might provide an answer.