The clinical signs, enteritis, weight depression, and hypoglycemia of spiking mortality syndrome were experimentally reproduced in broiler breeders and broiler chicks. Inocula included 1) virus-like particles from intestines of chicks with spiking mortality syndrome that had been banded in a discontinuous Renograffin gradient, 2) homogenized darkling beetles collected from litter of farms where spiking mortality syndrome had occurred repeatedly, and 3) homogenized embryos which had been inoculated with the Renograffin-banded material. Arkansas variant infectious bronchitis virus and arenavirus-like particles were identified in the inocula. Serology on samples from surviving chicks suggested the presence of an avian encephalomyelitis virus in one of the inocula. One-day-old (n = 172) and 2.5-day-old (n = 30) chicks were inoculated orally, and some were also injected intraperitoneally or subcutaneously, with 0.5 ml of the inocula. Twelve to fourteen days postinoculation, chicks were fasted for 4-6 hours, then briefly stressed with a cool water spray. Within 1.5 hours, inoculated chicks began dying with severe hypoglycemia and clinical signs of spiking mortality syndrome. Body weights were significantly depressed. Uninoculated controls (n = 130) from the same hatches, also fasted and stressed, were unaffected clinically and were not hypoglycemic. One group (n = 52) of inoculated chicks exposed to a controlled lighting program was unaffected clinically, had significantly higher mean plasma glucose levels, and had significantly less body weight depression than chicks exposed to continuous lighting. We concluded that exposure to controlled amounts of light/darkness can ameliorate much of the hypoglycemia, mortality, and runting-stunting associated with spiking mortality syndrome of chickens. The significance of the viruses and virus-like particles detected in the inocula is currently under investigation.