Quinolinic acid was administered intraperitoneally to male Wistar rats in a dose of 60 mmol, once daily for 8 days. The neuronal cell bodies in the paraventricular nucleus of the hypothalamus of quinolinic acid-treated rats exhibited electron microscopic features of increased cellular activity but some damage of neuronal cell bodies was also present. After quinolinic acid treatment, damaged cells predominated within the parvocellular division of the nucleus. The cells varied with respect to the degree of their damage. Besides, the cells exhibiting an increased activity, as well as those looking normally, were also present. In the magnocellular division of the nucleus, the cells exhibiting an increased activity predominated, although some forms of damage could also be distinguished.