We will describe measurements of the beam in the Fermilab Booster during the first five milliseconds. Most of the particle losses in the Booster are over after the first few milliseconds. At high intensity of4×1012the transmission is 75&percent;. Such high beam loss can be a limiting factor for future high repetition rate operation of the Booster. The evidence, although indirect, suggests that the losses are the result of incoherent space-charge effects at low energy. ©1998 American Institute of Physics.