Red opossums (Lutreolina crassicaudata) were trained in a Y-maze to locate a piece of food (the initial response, Ri) and afterwards to run back to the start box (the final response, Rf) where no reward was available immediately although a new trial was scheduled after a 30-s intertrial interval. Omission of food in some of the training sessions (Sessions 2, 10, 20, and 30) led to a decrement in latencies of the Rf, but only in Sessions 10 and 20, which was interpreted as evidence that primary

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... rustration elicited by omission of an expected reward increases the vigor of ongoing instru-1982). Primary frustration is also assumed to energize ongoing instrumental responses, an effect first reported by Amsel and Roussel (1952) . In their experiment, rats were exposed to a double-alley run-