Rats fixed with chronically indwelling bipolar electrodes pressed daily for direct electrical intracranial stimulation (lCS) of the hypothalamus. Rats pressed for a variety of intensities of ICS. As daily sessions with ICS continued, rats were given doses of morphine sulfate. The data confirm that morphine can accelerate pressing for ICS regardless of the intensities of ICS. One set of procedures indicated that programming ICS of about 75 J.tA (60-Hz sine waves of .25 sec) was sufficient to disrupt pressing previously sustained by intensities of 25 J.tA or lower. There were considerable individual differences in rats' reactivity to morphine; some showed marked acceleration of pressing while others showed little or no acceleration. These differences were not a direct function of intensity of ICS.