96 undergraduates learned either a single A-C list or successive A-B, A-C lists, after which 1/2 of the Ss in each condition were told that the experiment was finished and the remaining halves were asked to return in 1 wk. All Ss were then contacted by mail and asked to recall the 2nd-list materials. Significant proactive inhibition (PI) appeared when Ss were expecting to return to the experimental setting. No PI appeared when Ss believed the experiment to be finished. Ss reporting greater degrees of retention interval rehearsal (RIR) displayed greater amounts of PI. It is suggested that PI is determined not by competition at the time of recall, but by unlearning associated with RIR. (PsycINFO Database Record (c) 2006 APA, all rights reserved)