Three groups of 12 pigeons each were trained to discriminate between lights of 550 m,u (SD), correlated with 1-min variable-interval reinforcement and 570 m,u (SA), correlated with extinction. Group A was tested for wavelength generalization in extinction 1 min after meeting the discrimination criterion; Group B was tested 24 hr later; Group C was tested 24 hr later after a 3-min (reinforced) warm-up with the SD. The post-discrimination gradient of Group B was significantly flatter and showed significantly greater area shift than that of Groups A and C. The gradient of Group C was similar to that of Group A, indicating that the warm-up eliminated the effect of the delay period.The present study was designed to reconcile the discrepancy between the results of several earlier experiments on the effects of a delay period between training and testing for generalization.In 1958 Perkins and Weyant reported a study in which (response speed) generalization gradients obtained from rats one week after training on a runway were shown to be reliably flatter than those obtained immediately after training. Four groups were used, one tested immediately on the training stimulus, one tested immediately on the novel stimulus (alley and goal box of different color), one group tested after a one-week delay on the original alley, and one group tested after a one-week delay on the novel alley.Subsequently Thomas, Ost, and Thomas (1960) weeks after reaching a criterion, subjects were tested for generalization carried out in extinction in a manner introduced by Guttman and Kalish (1956). A wide range of spectral stimuli was successively presented with subjects' rate of responding to each providing the measure of generalization. There was no evidence that the three delay groups produced post-discrimination gradients which differed in any way whatever.There are many possible reasons for the discrepancy in the findings of these two studies. Different species were used, a different type of task, a different stimulus dimension, etc. Unlike Perkins and Weyant (1958), Thomas et al. (1960) tested for generalization after administering discrimination training. It is possible that one effect of this training was to increase the "stability" of the generalization gradient over a time period. In addition, Thomas et al. (1960)

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