The initial development of four vertical intracloud lightning flashes has been studied using time-resolved measurements of the electric field change from 7-8 ground locations at Kennedy Space Center, Florida. The initial field changes of the flashes were analyzed with a unidirectional piecewise-linear streamer model in an attempt to determine the direction and sign of the initial charge transfer. The results indicate that the discharges were initiated by an upward propagating negative streamer, rather than by a downward propagating positive streamer. The initial streamer appeared to progress upward at speeds between 1 and 3¿105 m/s for durations of 10-30 ms and deposited 1 to 4 Coulombs per kilometer of extent. These results disagree with those of earlier studies which used the shape of the overall electric field change to infer the sense of the streamer, and which gave relatively low values for progression speeds (104 m/s). The results agree with observations of VHF radiation source locations by Proctor in South Africa.