Four pups in a litter of eight Labrador Retrievers suddenly developed hind limb weakness. In three, paralysis ascended rapidly resulting in quadriplegia, cervical weakness, dysphagia and death. Postmortem examination revealed a severe polyradiculoneuritis in which roots, ganglia, and spinal and cranial nerves were heavily infiltrated by lymphocytes, plasma cells and macrophages and contained abundant protozoan pseudocysts. On sections of the brain and spinal cord protozoa were less frequent and appeared independent of the glial nodules which marked focal areas of necrosis. The organisms initially were thought to be Toxoplasma gondii, but this supposition was not supported by serological, immunocytochemical, or electron microscopic findings. Ultrastructurally the organisms resembled an unidentified sporozoan parasite, which has been reported in the CNS of dogs in Scandinavia. The inflamed spinal roots contained many degenerated and demyelinated axons. Electron microscopic studies indicated that the tachyzoite-like organisms, through their invasive and proliferative activities, brought about many of the degenerative changes in the Schwann cells and axons of the spinal roots and nerves.