The mechanism of CDD-induced appetite suppression was studied in male Sprague-Dawley rats. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) served as the model compound; additional experiments were performed using higher chlorinated CDDs (penta-, hexa-, hepta-, and a mixture) with substituents in the biologically crucial 2-, 3-, 7-, and 8-positions. All homologues, plus their equipotent mixture, led to an increase of plasma and brain tryptophan (TRP) levels and concomitantly serotonin (5-HT) turnover, in the very same dose range where acute toxicity occurred. The reason for elevated TRP levels was an inhibition of the activity of tryptophan 2,3-dioxygenase (TdO) in the livers of treated animals. A direct effect of TCDD on the central nervous system (CNS) was ruled out. Depletion of CNS 5-HT stores by administration of 5,7-dihydroxytryptamine (5,7-dHT) did not influence the TCDD-induced appetite suppression, and thus it was concluded that 5-HT causes aphagia by a peripheral mechanism. Based on the similarity of symptoms caused by CDDs on the one hand, and tumor necrosis factor (TNF) on the other hand, a mechanism of action is proposed where CDDs render the organism non-responsive to glucocorticoid receptor-mediated stimuli, locking the activities of several key enzymes of intermediary metabolism to low levels which do not allow the organism to respond appropriately to appetite suppression and hypoglycemia.