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% GeV1/2= s√E (sampling term) has been obtained with electron beams of up to 245 GeV: The uniformity of the response to electrons in an area of ∆n x ∆ϕ = 1.2 x 0.075 has been measured to be better than 0.8%.