A total of 384 Single Comb White Leghorn cockerels of the Hyline strain were fed either a basal (control) diet containing no added oil or a diet supplemented with 20% by weight of soybean oil, rapeseed oil from cultivars Tower, Candle, a mixture of Echo and Arlo high erucic acid rapeseed (HEAR), or R-500. Levels of erucic acid (22:1) in the rapeseed diets varied from .03 to 10.31%. Three birds from each unit were killed at 28, 56, 84, and 112 days and a wide range of tissues were examined histologically. A number of birds in all dietary groups had healed lesions of avian encephalomalacia. Two cockerels fed HEAR oil and 12 fed R-500 developed marked ascites, firm shrunken livers, hydropericardium, and cachectic muscular atrophy. Marked periacinar necrosis was present in birds dying with ascites. A significantly higher number of birds fed the rapeseed oils developed hepatic sinusoidal distention than birds fed the basal or soybean oil diets. Degenerative myocardial changes were seen only in R-500 and HEAR oil-fed birds. This, coupled with hepatic changes, producing shrunken firm livers, led to development of ascites, cachectic muscular atrophy, and periacinar hepatic necrosis. These changes were probably produced by the high erucic acid content of the HEAR and R-500 oils and the resultant grossly imbalanced diet.