The construction and performance of the barrel pre-series module 0 of the future ATLAS electromagnetic calorimeter at the LHC is described. The signal reconstruction and performance of ATLAS-like electronics has been studied. The signal to noise ratio for muons has been found to be 7.11+-0.07. An energy resolution of better than 9.5% GeV^1/2/sqrt{E} (sampling term) has been obtained with electron beams of up to 245GeV. The uniformity of the response to electrons in an area of Delta_eta x Delta_phi = 1.2 x 0.075 has been measured to be better than 0.8%