Motility of the anorectal area was studied in 19 patients with myotonic dystrophy and in 20 control subjects, before and after pudendal block. In patients with myotonic dystrophy, before anesthesia, resting pressure in the upper anal canal ( p less than 0.001) and duration and amplitude of relaxation of the rectoanal inhibitory reflex (p less than 0.01) were decreased. A myotonic contraction of high amplitude followed relaxation in all patients, but in control subjects this was not the case. In the lower anal canal, the duration of the rectoanal contractile reflex was prolonged as compared with control subjects (p less than 0.001). The pudendal block had no effect in the upper anal canal either on resting pressure or on amplitude and duration of the rectoanal inhibitory reflex, neither in patients nor in controls. After blockade, however, the myotonic contraction subsequent to the reflex in patients was significantly reduced in amplitude (p less than 0.01). In lower anal canal, the resting pressure was reduced to similar levels, both in patients and in control subjects, after pudendal block (p less than 0.01), and the rectoanal contractile reflex was abolished in both groups. This study demonstrates a number of functional abnormalities in the anorectal structures of patients with myotonic dystrophy. These abnormalities produce a decrease in resting pressure in the anal canal, and a reflex myotonic contraction subsequent to rectal distention due to both external and internal sphincter dysfunction.