Refractory periods of the substrates for lateral hypothalamic (LH) and medial pre-frontal cortex (MPFC) self-stimulation were behaviorally estimated. The beginning of recovery from refractoriness was estimated as the time at which recovery was 20% complete. In all 7 rats, this estimate differed substantially across sites, averaging 0.66 msec and 1.59 msec for the LH and MPFC substrates, respectively. The recovery of excitability approached asymptote later in the MPFC substrate (3.5 msec) than in the LH substrate (1.5 msec). These findings are consistent with the view that different fibers subserve the reinforcing consequences of LH and MPFC stimulation. This notion is strengthened by the observation that the rewarding effects of stimulation summated poorly when stimulating pulses were concurrently delivered to these two sites.