Strain differences and positive reinforcement with shock l Groups of 6, (j, and 7 IR and INR rats were adapted to test chambers in total darkness for one week, one month, and two months and habituated to operant bar apparatus for 70, 300, and 600 sessions then tested for operant level, shock contingent bar press, and extinction in ISO, ISO, and 350 sessions, all of 16-2/3 min. each. Shock proved positively reinforcing and cumulative shock durations were functions of shock intensity and animal reactivity (p < .001). Absolute response levels declined as a decelerated funct.ion of adaptation time. Results were interpreted in accord with an arousal and satiation model.