A number of researchers have verified the inhibitory effects of elevated H-2 concentrations on various anaerobic fermentation processes. The objective of this work was to investigate the potential for using hydrogen gas production to predict upsets in anaerobic digesters operating on dairy cattle manure. In an ammonia nitrogen overload experiment, urea was added to the experimental digesters to obtain increased ammonia concentrations (600, 1500, or 3000 mg N/l). An increase in urea concentration resulted in an initial cessation of H-2 production followed by an increase in H-2 formation. Additions of 600, 1500, or 3000 mg N/l initially resulted in the reduction of biogas H-2 concentrations. After 24 h, the H-2 concentration increased in the 600 and 1500 mg N/l digesters, but production remained inhibited in the 3000 mg N/l digesters. Both methane and total biogas production decreased following urea addition. Volatile solids reduction also decreased during these periods. The digester effluent pH and alkalinity in creased due to the increased NH4+ formed with added urea. Based on these results, changes in H-3 concentration could be a useful parameter for monitoring changes due to increased NH3 in dairy cattle manure anaerobic digesters. (C) 2001 Elsevier Science Ltd. All rights reserved.