We describe a head-mounted micropump-injection system designed for the infusion of nanoliter volumes of drug solutions into discrete brain regions of the freely moving rats. Using a miniature step motor, the micropump-injection system can be readily constructed from commercially available supplies. In calibrating the micropump-injection system, we found that it will deliver a reliable volume of 50 nl per infusion over a 1-h period, with an infusion given every 1 min. From in vivo testing, we also found that rats readily self-administered up to 100 infusions of D-amphetamine into the nucleus accumbens at regular intervals, suggesting that this system can deliver constant volumes of infusions over time in freely moving rats. It (1) attaches easily to an implanted guide, (2) is compact and durable, (3) weighs only 10 g, and (4) is well tolerated with no apparent discomfort to the animal. This system overcomes some of the weaknesses of currently used intracranial self-administration systems.