Truly random sequences of tone, conditioned stimuli (CSs), and shock, unconditioned stimuli (USs), were given to groups of rats; then conditioning to the OS was measured using a conditioned suppression procedure. In Experiment 1, eliminating chance CS-US pairings by systematically removing CSs that overlapped USs weakened conditioning monotonically as a function of the number of paired CSs removed. In Experiment 2, systematically delaying early chance pairings, while holding constant the number of CSs, USs, and pairings, produced a nonsignificant weakening of conditioning. In Experiment 3, delaying pairings again produced a nonsignificant weakening of conditioning when the early nonpaired events were CSs, but significantly weakened conditioning when the early nonpairings were USs. The data suggest that each chance pairing in the truly random control produces an increment in conditioning unless "blocked" by prior USs alone.The truly random control is a procedure frequently used to control for nonassociative factors in Pavlovian conditioning (e.g., Ayres & Quinsey, 1970;Bull & Overmier, 1968;Rescorla, 1968). It is characterized by the presentation of conditioned stimuli (CSs) and unconditioned stimuli (USs) randomly and independently of each other so that CS onsets provide no information about the occurrence of US onsets. Because there is no contingency between the CS and US in such a procedure, it was originally thought that the procedure should produce little or no conditioning (Rescorla, 1967). However, in several recent experiments the truly random control has apparently produced very powerful conditioned effects (Benedict & Ayres, 1972;Kremer, 1971;Kremer & Kamin, 1971;Quinsey, 1971). The most recent of these studies, moreover, showed that an important determinant of the conditioned 'This research was supported by Grant BG-36982 from the National Science Foundation and by a grant from the Research Council of the University of Massachusetts. We wish to thank William Matthews, who collected the data in Experiment 3, and Dan Anderson for his valuable comments on the manuscript. We also wish to thank Western Washington State College, Bellingham, Washington, for its support during the preparation of the manuscript.a Requests for reprints should be sent to John J.