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In an investigation over five months the adrenal toxic effect of the quinoxaline derivate olaquindox in pigs of different ages (10 and 16 weeks) after accidental feeding of a 15-fold overdosage of olaquindox is demonstrated. The substance induced clinical and hematological changes comparable with Addison's disease. A reduced gain of body weight was observed, predominant hematological findings were a hyperkalemia and a hyponatremia, which reversed to normal values after 17 weeks in the younger group and after 14 weeks in the older group. Whereas five weeks after the first intake of the feed, degenerative changes of the zona arcuata were most prominent, a destructive fibrosis and a reduction of the adrenal cortex were more marked 12 and 13 weeks later. Due to the loss of aldosterone the electrolyte imbalances led to degenerative changes in the skeletal muscles and in the myocardium. Pigs of younger age were more affected than elder ones. Subcapsular cells with basophilic nuclei resembling normal cells of the zona arcuata appear 18 to 22 weeks after the first overdosage of olaquindox and are suggestive in combination with normal electrolyte values of a regeneration of the zona arcuata.