Reinforced 1 group (N = 6) of curarized Sprague-Dawley male rats for heart-rate increases and another group (N = 6) for heart-rate decreases by electrical stimulation of the medial forebrain bundle. All Ss were run for 4 sessions each of which consisted of an adaptation, conditioning, and extinction phase. During the conditioning phase of each session a feedback stimulus was presented whenever S's heart rate met some criterion. 25 successive-criterion heartbeats produced the reinforcement. The criterion was made more stringent as S's heart rate changed in the appropriate direction. The heart rates of the 2 groups diverged progressively over the 1st 3 conditioning phases and converged during extinction. (16 ref.) (PsycINFO Database Record (c) 2006 APA, all rights reserved). © 1969 American Psychological Association.