A morphological study of the beta-cell of the pancreas in rats chronically exposed to polychlorinated biphenyls (PCBs) is presented. In rats who received 200 p.p.m. PCBs in their drinking water for 13 months, marked dilatation of rough endoplasmic reticulum (RER), vesiculation of RER, hyperplastic Golgi complexes, a reduced number of secretory granules, an increased number of mitochondria and mitochondrial changes were observed. Numerous acinar-beta and beta-acinar cells were also present. These structural features suggested an increased activity of the beta-cells in these animals. Since in previous studies we found an increase in the activity of the adrenal zona fasciculata of rats exposed to PCBs, the present findings were considered a result of solicitation of beta-cells for carbohydrates' homeostasis.