The prompt neutron lifetime of the SRE was measured by both the oscillation and random noise techniques. Measurement by use of the oscillation technique gave a prompt neutron lifetime of (5 25 plus or minus 0 35) x 10/sup - 4/ sec for a calculated beta of 7 x 10−3. The measured noise response indicated a lifetime of (5.25 plus or minus 0.7) x 10−4 sec. Both measured values are in agreement with the calculated value of 5 x 10−4 sec. Four experiments utilizing the noise analysis technique were performed to determine the prompt neutron lifetime of the KEWB. All four experiments gave results which agreed within 3%, For an estimated beta of 8 x 10−3, the measured value obtained was (7.8 plus or minus 0.3) x 10−5 sec. This is in reasonable agreement with both the energy independert calculated value of 6.6 x 10−5 see and the value of 6.2 x 10−5 sec obtained from the experimental inhour equation The oscillation technique has been found to be better suited for lifetime determinations in reactors where the prompt neutron break frequency is less than 5 cps. Reactor noise analysis is more suitable for reactors which have prompt neutron lifetime break frequencies above 20 cps. (auth).