Systemic treatment with morphine has been shown to facilitate intracranial electrical self-stimulation reward elicited from the ventral tegmental area (VTA) as was determined using a response rate-insensitive threshold measurement. In the present experiment graded doses of morphine were microinjected into the mesolimbic system to determine the site of this morphine action. Morphine injected into the nucleus accumbens did not affect the threshold and response rate of self-stimulation by electrodes in the VTA, while relatively high doses of morphine injected into the VTA produced a long-lasting decrease of the threshold of self-stimulation by electrodes in the nucleus accumbens. It is concluded that morphine can facilitate self-stimulation when injected into the VTA, and that a concerted action of morphine on multiple brain sites may be involved in the interaction of the drug with brain reward.