We compared the activity of Ib spinal interneurons in five patients with progressive supranuclear palsy (PSP) with six age-matched control subjects. Stimulation of the medial gastrocnemius nerve at motor threshold intensity activated Ib afferents that in turn inhibit H reflexes from the soleus muscle. Maximum inhibition occurred at interstimulus intervals of 6 and 8 ms for both control subjects and PSP patients and was significantly greater in the PSP patients. Increased Ib activity of PSP patients may be caused by loss of inhibition of Ib interneurons through degeneration of the medullary reticulospinal pathway. The corticospinal pathways, unopposed by the medullary reticulospinal tract, may excessively activate Ib internurons.