Lever contacts of mice were measured for 1 hr. in a Skinner box situation. In the first 30 min. different groups were exposed to darkness, continuous light, and regularly and irregularly blinking light; in the final 30 min. light was contingent upon lever contact. Preexposure to light reduced the number of later lever contacts; the reduction was greater for the Blink groups than the Continuous Light group. (PsycINFO Database Record (c) 2006 APA, all rights reserved)