Experimental studies on brain self-stimulation in animals have concerned themselves primarily with the influence of locus of electrode placement and the intensity of the electrical stimulus ( 1, 2, 4, 5, 6) .2 In the present investigation we are interested in studying the possible differential effects of (a) electrical sine waves of 10, 35, 50, 60, 100, 300, 2000, and 3000 cycles per second (cps); and (b) rectangular electrical pulses of 2 msc. duration at repetition rates of 10, 60, 100, and 300 pulses per second (pps). The peak intensity of the stimulus was kept constant and equal for the sine waves and the rectangular pulses. METHOD Apparatus The sine wave stimuli were delivered to the animal as the output from a Hewlett-Packard oscillator. The output was monitored by an oscilloscope in the output circuit. Intensity of stimulus was maintained constant for all animals at 40-microamp. peak value. Silver, twisted bipolar electrodes were used. These were mounted in a lucite base, secured to the skull by means of jeweler's screws (4). The electrodes were implanted stereotoxically in the hypothalamic area (Kreig co-ordinates) and were insulated except at the tip cross-sections. Distance between the tips was less than 1 millimeter. Histological verification is in process, but has not been completed as yet. S was placed inside a wooden Skinner box and was visible to E by means of a mirror fitted at the top of the receptacle. Light flexible wires led down to the electrodes. Depression of a platform-type lever delivered the stimulus and activated one or both of two timers which recorded cumulatively the duration of and the duration of all presses, respectively. The circuit also contained two counters which recorded the number of re-enforced presses and the total number of presses, respectively. Graphic records of cumulative presses were obtained by means of a Davis recorder. Electrical stimulation was applied on a variable ratio schedule, with an average of one stimulation every four presses. Electrical current to the rat's brain was delivered as long as the platform remained depressed.