Data by the author, as well as from other laboratories, show residual fear as the main source of intertrial responses (ITR) during defensive instrumental training in rats, cats and dogs.. Between-subject differencies in ITR rate arise in the course of training. The differences depend on training conditions and on subject's emotionality. Under the same conditions higher ITR rates correspond to more rapid avoidance learning. The increase of ITR rate is regularly observed in the following circumstances: at early stages of training, after transfer to a less salient discriminative stimulus, and after a change of the taskrequirements. The decrease of ITR rate corresponds to the plateau level of avoidance or escape performance. Both a lower ITR rate and reduced flexibility of behavior are found after lesions of the amygdala nuclei.