Expressions for the reactivity effects produced by the motion of a fluid in a reactor core are developed using perturbation techniques. It is shown that in a fixed fuel reactor significant reactivity effects can be produced by the “drag” of the fluid on the neutron population. In a fluid fuel reactor, the transport of the delayed neutron precursors can produce reactivity changes approaching a dollar for very moderate fluid velocities. An expression for the delayed neutron lifetime in a fluid fuel reactor is developed, and the corresponding inhour equation is derived.