As a part of a program of reactor safety investigations, the response of a heterogeneous, water-moderated and -reflected reactor (SPERT I) to instantaneous reactivity additions has been studied experimentally with initial temperature of 20°C and initial power level of 5 watts. Excess reactivity additions from approximately O.3% to 1.4%, which result in asymptotic reactor periods from 10 sec to 7 msec, produced self-limiting power bursts with peaks up to 1300 Mw. Plots of the typical behavior of reactor power, fuel plate temperatures, and transient pressures for these tests are presented and discussed. Maximum reactor power, fuel plate temperature, pressure, energy release, and other quantities are correlated as functions of reactor period. The instantaneous excess reactivity of the system during the transient test has been computed from the experimental power behavior and typical results are shown. The reactivity compensation necessary to limit a power burst of this type has been determined and is discussed as a function of initial reactor period. Several mechanisms for the self-shutdown of the reactor are postulated and discussed in light of the experimental results.