Paired-pulse stimulation techniques were used to infer distributions of refractory periods (RPs) for the directly activated medial forebrain bundle (MFB) substrates of brain stimulation reward (BSR) and stimulation-induced feeding (SIF). Each RP distribution suggested (a) a subpopulation of contributing fibers with refractory periods between 0.4 and 0.6 ms, (b) absence of significant numbers of fibers with refractory periods between 0.6 and 0.7 ms, and (c) a second subpopulation (or set of overlapping subpopulations) with refractory periods between 0.7 and 1.5-2.5 ms. Similar RP estimates were obtained in different animals, despite the fact that stimulation sites ranged from the anterior lateral hypothalamus to the ventral tegmental area; this finding is consistent with the view that the directly activated substrate of both behaviors involves MFB fibers of passage. While the possibility cannot be ruled out that the two behaviors result from activation of distinct sets of fibers with remarkably similar refractory periods, the more likely possibility is that a common--or at least partially common--MFB substrate underlies brain stimulation reward and stimulation-induced feeding.