As CS complexity (two levels serial, two nonserial) increased, the intertrial responding of Sprague-Dawley rats decreased. 'Learner' rats did much more intertrial responding than did 'nonlearners,' and their responding correlated positively with-but had no direct effect on the absolute level of-their conditioned avoidance responding, increasing over trials and then tapering off. The general findings, replicated with Iowa hooded rats, imply that intertrial responses are generalized avoidance responses contingent upon CS-US pairing and subject to CS discriminability. Responses during the intertrial interval that are not differentially reinforced by the experimenter are frequently referred to as 'spontaneous' or intertrial responses. Sheffield (1965) referred to them as an ever-present nuisance, implying that they make it less easy to determine the precise extent of conditioning. Most investigators either ignore, or expend considerable effort to minimize, the occurrence of those responses, but little is known about the factors responsible for eliciting them. In an attempt to make theoretical sense of intertrial responses, Mowrer and Lamoreaux (1951) suggested that they simply reflect the ongoing process of discrimination learning involved in a conditioning procedure. Such a notion implies that the intertrial responses are generalized avoidance responses elicited by the fear associated to the background cues of the apparatus. The extent of this generalization may be considered a function of the difficulty the subject has distinguishing situations with the conditioned stimulus (the CS, in the CS-US interval) from situations without the CS (intertrial interval). It would be expected, then, that intertrial responding should increase early in training, when that distinction is low, and decrease as the conditioned avoidance response, and the discrimination of the CS, becomes well established-a trials effect. Such tends to be the general finding in the literature (Black and Carlson, 1959; Brush, 1957; Church, Brush, and Solomon, 1956; Kamin, 1954; Thompson, Sachson, and Higgins, 1969). Another prediction that would follow from Mowrer'?s generalized-re