We present an incidence of acute nitrate-nitrite poisoning in a guinea pig (Cavia porcellus) colony which resulted in the substantial and spontaneous death of 28 out of 88 guinea pigs. The cause of death was traced to nitrate-nitrite toxicity from ingestion of the green leaves of overfertilized Cichorium intybus. The predominant symptoms in the 28 guinea pigs were chocolate brown bloody discharge from all natural orifices, cyanotic mucous membrane, mydriasis, and bloated abdomen. Necropsy of all the 28 animals showed hemorrhage in all visceral organs grossly, and tubular nephrosis of the kidney, lymphoid depletion in the spleen, hydropic changes and necrosis in hepatocytes, and hemorrhages in the intestine and lungs histopathologically. Nitrate-nitrite toxicity was confirmed spectrophotometrically. The toxicological analysis of the feed samples of C. intybus revealed 2.1 g% nitrate, 1.0 g% nitrite, and 80 ppb citrinin. The intestinal loop had 10 ppm of nitrate and 1 ppm of nitrite. To the best of our knowledge, this is the first report of acute nitratenitrite poisoning in guinea pigs owing to the ingestion of the green leaves of overfertilized C. intybus.