A report of two experiments on the depressant effect of stimuli upon an on-going bar pressing response in albino rats. Illumination by means of a light bulb was paired with grid shock with intervals of 0, 1, 4, 7, 10, 20 and 30 sec. from the onset of the light in experiment 1, and 0, 5, 10, 20, 40 and 80 pairings of 7 sec. interval with shock in experiment 2. Maximum depressant effects were found at the 20 sec. interval and for 40 pairings of light & shock during a 10 minute test period. Explanations for the presence of a maximum effect in number of pairings are (1) a possible "adaptation" effect of shock, and (2) partial avoidance of the shock stimulus. 16 references. (PsycINFO Database Record (c) 2006 APA, all rights reserved)