The guinea-pig myenteric plexus-longitudinal muscle (MPLM) preparation electrically stimulated at 0.1 Hz is very sensitive to the inhibitory effects of opiates. We used this preparation to detect an inhibitory response (IR) which was produced by electrical stimulation at 5 to 20 Hz. The magnitude and duration of the IR are determined by the parameters of the stimulation, mainly by the frequency and duration of the period of stimulation. Maximal IR is obtained with symmetrical biphasic stimuli of 2 msec duration and supramaximal voltage at 20 Hz applied for a period of about 5 min. The IR is calcium-dependent, cannot be attenuated by washing and is mediated by several components. About 55 to 70% of the IR can be reversed by specific narcotic antagonists and therefore it is considered to be produced by the release of endorphins. There are at least two additional components, one small, adrenergic in nature, and a third one which has not been identified. The offset rate of the IR is measured in minutes, while enkephalin and human beta-endorphin have half-lives of 10 and 85 sec, respectively, after washing. This suggests that the endorphin that mediates the opiate component of the IR may be a different one with slower offset rates. Myenteric plexus-longitudinal muscle strips obtained from guinea pigs which were made tolerant to morphine by subcutaneous implantation of three pellets were cross-tolerant to the opiate component of the IR elicited by electrical stimulation.