- Several types of short-term postactivation potentiation (PAP) effect were examined in limbic forebrain pathways in the chronic rat. We tested 9 different stimulation sites and 1-3 target sites for each stimulation site. All pathways showed PAP effects following activation by single electrical pulses or pulse trains. Using exponential curve fitting procedures, we found that the decay curves could be best fitted by one, or a sum of 2-3, exponential curves. On the basis of time constants, these curves fell into 4 different categories: facilitation (tau = 80 ms), augmentation (tau = 7s), potentiation 1 (tau = 70s), and potentiation 2(tau = 6.5 min). The latter component was the one most reliably generated in the chronic preparation. Frequency facilitation (facilitation during a stimulation train) was also examined and it appeared to be based upon a mechanism similar to that underlying paired pulse facilitation. Evidence is presented that facilitation and augmentation may be based on the the same mechanism. The possibility that the remaining components involve different mechanisms is discussed.