An integral equation for the time‐dependent reaction is derived on the basis of the two‐group model of neutron multiplication. When the effective multiplication constant is given, the solution reduces to a determination of the mean timeTbetween consecutive generations, which is largely controlled by the circulation of neutrons in the reflector. If the steady distribution is known,Tcan be calculated from a simple formula.To illustrate the orders of magnitude which are involved,Twas calculated for a small pile surrounded by an infinite reflector. A value ofT=450 microseconds was obtained assuming a mean lifetime in the core of 50 microseconds and a mean lifetime in the reflector of 14 000 microseconds.