To determine the functional changes occurring in the early period after the development of chronic cord compression, an ossification of the ligamentum flavum model, which is a model of chronic cord compression, was created in domestic rabbits using bone morphogenetic protein, and monitored evoked spinal cord potentials. Sixty-seven rabbits were used for these experiments. Myelography, evoked spinal cord potential measurements, and histologic examinations were performed in 16 of the ossification of the ligamentum flavum rabbits and six of the healthy control rabbits. Our results show that 1) mild degrees of cord compression produce greater evoked spinal cord potential alterations in chronic than in acute compression, as reported in the literature; 2) chronic compression from the dorsal side produces injury in the superficial layer of the dorsal column and dorsolateral column; and 3) changes in evoked spinal cord potentials precede the development of paralysis and histologic changes.