Nitrification rates in two types of animal slurry were measured at temperatures between 20 and 60 degrees C. the rates were assessed in rapid laboratory assays using samples from aeration tanks of large-scale treatment plants for pig or veal-calf slurry. Maximum nitrification rates for the two slurries were observed around 35 and 40 degrees C. For veal-calf slurry, the observed maximum rate was 1.5 mmol NH4+ per gram volatile suspended solids per hour. The maximum rate for digested pig slurry was 0.11 mmol g(-1) h(-1)). For digested pig slurry, a sharp drop in nitrification rate was observed at temperatures higher than 40 degrees C. The rate in veal-calf slurry dropped at temperatures higher than 45 degrees C. No nitrification was observed above 45 degrees C in digested pig slurry and above 50 degrees C in veal-calf slurry.