Behavioral, polygraphic, biochemical and histological aspects of thiamine deficiency in rats induced by thiamine-deficient food and pyrithiamine treatment (40 mg/kg daily for 4 days) are described. Behavioral alterations were essentially characterized by ataxia, pilo-erection and paresis. Polygraphic data indicated an increase in slow-wave sleep (SWS) of 33% and decreases in paradoxical sleep (PS) and wakefulness (W), respectively, of 69% and 27%. These effects were reversed by complete food and thiamine administration, the reversal including an overshoot in PS. Biochemical assays, performed when the polygraphic data indicated a large effect, demonstrated a significant increase in serotonin (5-HT) and 5-hydroxyindolacetic-acid (5-HIAA). These effects were particularly evident in the raphe system and the locus coeruleus. Histological data from the raphe dorsalis displayed a notable increase in yellow fluorescence in pyrithiamine-treated animals over controls. We conclude from these experiments that a deficiency in thiamine affects the serotonergic system and that the subsequent effects on sleep are a consequence of this serotonergic change.