Exteroceptive suppression (ES) of temporalis muscle activity was studied in a group of normal volunteers and also in a group of patients with fibromyalgia but without headache. The results for each patient were within normal limits as depicted from the normal group (Mean +/- 2SD). The average duration of "late ES period (ES2)" of temporalis muscle in patients with fibromyalgia was shorter than that of normals. When all the subjects in the study population were assigned into two groups according to presence of sleep disturbances, a delayed average ES2 latency and a diminished average ES2 duration was observed in the group of subjects with sleep disturbances. Average ES2 latency was found to be delayed in the group of subjects without dysmenorrhea. Presence of differences in ES parameters of temporalis muscle in fibromyalgia may point to a disturbance of antinociceptive brain stem mechanisms in this disease.