13 rats were prepared with chronically indwelling electrodes, the exposed tips of which were in the vicinity of median forebrain bundle. In addition, 7 of these received septal lesions, 2 were inflicted with unilateral hippocampal lesions, and 2 sustained unilateral lesions of the anterior neocortex. The remaining two Ss received no further surgery beyond the electrode implant. All were trained to bar press for brain stimulation at crf, FI 10-sec., FI 20-sec., FI 40-sec., FI 60-sec., FI 90-sec., and FI 120-sec. reinforcement contingencies, respectively. Although the major outcome of the study was that bar pressing could be developed and maintained by FI ICSS in a manner similar to FI food reinforcement, there were several characteristics of the FI ICSS-controlled behavior which distinguished it from bar pressing controlled by FI food reward. (1) Even when an S had been exposed to FI ICSS for as many as 28 hr., it was necessary at the beginning of each daily session to “prime” and “retrain” S. (2) For FI 60 sec. to FI 120 sec. the over-all rate and coarse grain of the cumulative record of the ICSS sustained behavior was less pronounced than that characteristic of food-controlled fixed-interval. (3) The lesions situated in the septum, hippocampus, and cortex did not alter the FI performance to a noticeable extent.