The influence of temperature, 50 and 60 degreesC, at hydraulic retention times (HRTs) of 20 and 10 days, on the performance of anaerobic digestion of cow manure has been investigated in completely stirred tank reactors (CSTRs). Furthermore, the effect of both daily downward and daily upward temperature fluctuations has been studied. In the daily downward temperature fluctuation regime the temperatures of each reactor was reduced by 10 degreesC for 10 h while in the daily upward fluctuation regime the temperature of each reactor was increased 10 degreesC for 5 h. The results show that the methane production rate at 60 degreesC is lower than that at 50 degreesC at all experimental conditions of imposed HRT except when downward temperature fluctuations were applied at an HRT of 10 days. It also was found that the free ammonia concentration not only affects the acetate-utilising bacteria but also the hydrolysis and acidification process. The upward temperature fluctuation affects the maximum specific methanogenesis activity more severely as compared to imposed downward temperature fluctuations. The results clearly reveal the possibility of using available solar energy at daytime to heat up the reactor(s) without the need of heat storage during nights, especially at an operational temperature of 50 degreesC and at a 20 days HRT, and without the jeopardising of the overheating. (C) 2004 Elsevier Ltd. All rights reserved.