In each of two experiments, different groups of pigeons were required to discriminate between one of two basic kinds of stimulus differences: stimulus quality or stimulus location. For stimulusquality groups, a key was illuminated by one of two colors on trials ending with food delivery and by the other color on trials ending with no food. For stimulus-location groups, a key was illuminated at one of two locations on trials ending with food delivery and at the other location on trials ending with no food. The birds began to respond differentially to the stimuli (i.e., peck the keys on food trials and not peck the keys on no-food trials) earlier in acquisiton if the stimulus qualities served as the signals for trial outcomes than ifthe stimulus locations served as those signals. The results from both experiments are consistent with predictions from a hypothesis regarding interactions among the qualities and locations of stimuli and responses (the "qualitylocation hypothesis"). Furthermore, the present results support other recent demonstrations of the important role that spatial relations among stimuli can play in classical conditioning.Previous research concerning the role of spatial relations in classical conditioning has focused on the necessary and sufficient conditions for the acquisition of signal value by a CS (e.g., Marshall, Gokey, Green, & Rashotte, 1979;Rescorla & Cunningham, 1979;Testa, 1975). Those studies asked how the spatial relations between a CS and a US or between a first-and a secondorder CS can affect the development and expression of a conditioned response. Another way in which a spatial relation might be an important factor in classical conditioning, however, is suggested by Szwejkowska's (1967) experiment and by some informal observations made by Rowe, Miller, and Green (1984) during a series of instrumentaI-eonditioning experiments. Both Szwejkowska's andBowe et al.'s studies involved an effect of spatial relations on the acquisition of a discrimination between two stimuli, rather than on the acquisition of a conditioned response to a single stimulus. In both cases, differential responding was more readily established if the stimuli differed in "quality" (i.e., sound