Most previous research on the effect of the duration of preceding discrimination training on responding to a new stimulus has measured the responding during extinction. To reduce effects originating in the extinction procedure itself, the present study assessed the effect of discrimination training on responses to a new negative stimulus added during continued discrimination training. Pigeons were given a new negative stimulus (blue key) after 0, 1, 3, or 9 days of discrimination training with a yellow key as the positive stimulus, and both a green key and a red key as negative. Fewer responses were made to the blue key when it was introduced after nine days of discrimination training than after less discrimination training. That effect of long discrimination training agrees with reported results from extinction tests. However, the effect of briefer discrimination training in the present study differed from reported results with extinction testing. It appears that testing during continued discrimination training eliminates a distortion present in extinction tests of the effect of discrimination training on responding to a new stimulus.