We report on measurements of the time dependence of the populations of Rydberg states of Na with n=25-33 produced by stepwise excitation or deexcitation from the initially populated state 28s, stimulated by room-temperature blackbody radiation. We also present a theoretical modeling of the experiments, which mainly consists of solving the set of coupled rate equations for all the states involved. The results of the calculations show that for times of the order of the lifetime of 28s the populations of ns and nd states produced via two-step transitions can be up to 30% of the populations of np states. We also find that np states get repopulated via three-step transitions at the 2-12 % level. A comparison between the data and calculations shows good agreement. © 1995 The American Physical Society.