The effects of 3,3',4,4'-tetrachloroazobenzene (TCAB) and 3,3',4,4'-tetrachloroazoxybenzene (TCAOB) on lymphoid organs in male Sprague-Dawley rats were investigated in various acute exposure studies. Significant thymic atrophy was observed in rats 10 days after the i.p. administration of either compound (on days 1 and 5) at a dose of 25 mg/kg. When 8-week-old animals were studied, the relative thymus weight was reduced to 69% and 49% of the control value by the respective treatment of TCAB and TCAOB. In 2 groups of weanling rats the same dosage of TCAOB was able to reduce the relative thymus weight to 31% and 38% of the comparable control animals. In addition, TCAOB causes a decrease in the body weight gain and a decrease in the weights of major organs in the weanling animals. This toxic response cannot be explained solely on the basis of decreased food intake since qualitatively the same results were observed in a pair-feeding experiment. The involvement of glucocorticoid hormones was rejected as the underlying mechanism since adrenalectomy was found to provide no protection towards the degenerative effect seen upon the lymphoid tissues. This investigation constitutes the first study concerning the effects of these 2 environmental and occupational toxicants on lymphoid organs.