Studied the effect of response effort on rate of responding and choice behavior in a 2-bar situation in 2 experiments with 17 male Sprague-Dawley rats. Exp I used a limited test session and tested at a single FR ratio. Exp II employed a free behavior situation, in which total daily water intake was obtained by barpressing. Ratio requirements beginning with continuous reinforcement and followed by sessions of FR 5, 10, 20, 40, 80, 120, and 160 were tested. Relations between response rate and preference behavior were schedule dependent. On low-ratio schedules the highest response rates and preference were for the light bar. As the ratios were increased, however, response rates became higher and preference was switched to the heavy bar. (19 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved)