Information was sought on how the strength of fear, acquired and extinguished in a classical conditioning paradigm, might be affected by certain ofthe circumstances that normally are associated with the course of avoidance learning. Following preconditioning exposures to the conditioned stimulus, one group of rats (Group PRF) was given continuous conditioned stimulusunconditioned stimulus (CS-US) acquisition trials followed by stepwise reductions in US probability (given the CS) over three phases to about 11 % for the final phase. Another group, Group US(lo), was given continuous CS-US pairings throughout, but, following acquisition, received stepwise reductions in US intensity (to permit evaluation of a progressively changing feature of the US over a wide range without having to employ excessive shock) over the same phases. A third group, Group US(hi), received unchanging (from acquisition) CS-US pairings over these phases, and an explicitly unpaired control group (Group RU) was included. Although outcomes differed somewhat depending upon whether suppression ratios or absolute measures of responding were considered, the major findings were that suppression to the CS was complete by the end of acquisition and persisted thereafter throughout the three phases for all but control subjects. In contrast to Group US(lo), which showed relatively little resistance to extinction of suppression to subsequent CS-only exposures, Groups US(hi) and PRF displayed marked resistance over two separate, extended sets of extinction sessions. Suppression to context cues was pronounced only for Groups US(hi) and RU, and varied as a function of US parameters and not whether the shocks were or were not signaled. Theoretical reconciliation of these findings was most difficult for Group PRF.The concept of acquired fear, which is generally presumed to result from classical conditioning procedures (Brown, 1961;Kimble, 1961), has been widely invoked in accounts of signaled avoidance learning (Mowrer, 1947(Mowrer, , 1960Rescorla & Solomon, 1967). To many, the paradigmatic similarities of fear-conditioning and avoidance-learning procedures have been a sufficient basis for theoretical speculation involving common explanatory mechanisms. Thus, it has been postulated that fear becomes conditioned to the signaling stimulus early in avoidance training as a result of pairings with shock, and then serves motivationally to energize and reinforce avoidance responding.Clearly, such theorizing depends critically upon implicit assumptions about the course of fear acquisition and extinction during avoidance learning, not to mention suppositions regarding the possible interaction of fear and avoidance behavior. However, it is not commonly recognized that two-factor accounts of avoidance learning really rest upon two separate kinds of assumptions about fear acquisition and extinction. On the one hand, two-factor theory implies that fear would develop and be maintained in a classical conditioning paradigm in which parameters were fashioned after features of ...