Response Force as a Function of Amount of Reinforcement.

5 groups of 6 rats each were trained to press an isometric lever and were rewarded with either 1, 2, 3, 4, or S 20-mg. pellets on responses meeting or exceeding an 8-gm. force criterion. After 10 Acquisition sessions all Ss were shifted to 3 pellets for 6 further sessions and then received 2 Extinction sessions. Both force and variability of bar pressing varied inversely with amount of reward by the end of Acquisition, but not during Shift or Extinction. An account of these results in terms of criterion discrimination is suggested.