The pulsed nature of energy release in reactors creates specific reactivity effects. In addition to the already known features of pulsed reactors (the inevitability of power fluctuations with a frequency equal to half the pulse repetition frequency, positive feedback from external periodic fluctuations in reactivity, the effect of dynamic bending of fuel elements), discussion is given to the phenomenon of periodic fluctuations in the coolant temperature (and, accordingly, in reactivity), the frequency of which depends on the coolant flow rate. An estimate of the amplitude of fluctuations for the reactors at FLNP JINR is presented.