If a burst of neutrons is injected into a reactor, the rate of prompt decay of the resultant flux should be proportional to the reactivity in dollars as measured from prompt critical. The proportionality constant is the ratio of the effective delayed neutron fraction to the prompt neutron generation time and can be determined by pulsing the reactor at (delayed) critical. Experiments with distributed poison in several highly enriched, hydrogen-moderated critical assemblies indicate that such pulsed reactivity measurements are reliable at least as far as