1. The latencies of the visual evoked responses, indices of central nerve conduction, and peripheral nerve conduction were slowed in patients with primary hypothyroidism compared with controls. 2. In thyrotoxic patients, there was no change in the latencies of the visual evoked responses and peripheral nerve conduction compared with the control group. 3. The abnormalities seen in hypothyroidism were reversed by L-thyroxine therapy. 4. Warming untreated hypothyroid patients significantly improved both central and peripheral nerve conduction. 5. The conduction delay found in hypothyroidism is to a large extent dependent upon a subnormal body temperature.