Three cases of accidental acute fertilizer poisoning in cattle resulted in substantial death loss. Water hauled in tanks previously contaminated with a nitrogen-based fertilizer was the source in all cases. In 2 cases, analysis of the water measured urea at 1,640 and 2,300 ppm and ammonia-nitrogen at 640 and 750 ppm, respectively. Confirmation of urea toxicosis was made by measurement of toxic levels of ammonia-nitrogen in the rumen contents (>800 ppm) and blood (>8.0 ppm). In Case 3, no urea was detected in the water but ammonia-nitrogen was measured at 1,670 ppm and nitrate at 1.1%. Toxic levels of nitrate were measured in ocular fluid from 2 animals (60 ppm). Rumen contents contained toxic levels of ammonia-nitrogen (300 ppm) and a pH of 8.7. Interpretation of ammonia levels in rumen contents should take into account the source of ammonia as well as the pH since urea is more toxic than some ammonium salts. When the source is unknown, analysis of blood and/or serum ammonia-nitrogen instead of rumen content is recommended. The rapid progression of signs to death with ammonium nitrate poisoning may explain the lower levels of nitrate attained in ocular fluid than observed with pure nitrate intoxications.