In the differential outcomes procedure (DOP) the organism is exposed to a stimulus (S1) in the presence of which a response is required (R1), producing a consequence or reinforcer (r1). In the presence of a stimulus S2, a response R2 is required, producing a different reinforcer (r2). The effect is an increment of the speed of acquisition and final precision in conditional discrimination tasks. A two factors theorybased explanation establishes that the use of differential outcomes involves the activation of expectations of reinforcer through a Pavlovian mechanism. The presence of these differential outcomes promotes the activation of a system of implicit memory in the solving of conditional discrimination tasks. According to this argument the expectation of reinforcer can function like a sign that guides the response of organism. Recently the effect of DOP has been demonstrated in situations where what is learned isn’t directly observed, as is the case when a retention interval in retroactive interference is manipulated.