1. The present study compares the direct actions of morphine on two brain sites known to be rich in opiate receptors, namely, the caudate nucleus and the central gray. Recordings and morphine injections were made through a multibarrel glass micropipette using microiontophoresis. 2. Four different patterns of neuronal response to increasing currents of morphine were recorded in both brain regions. 3. Differences in the response to morphine between the two sites were detected in morphine-dependent rats. While the caudate neurons exhibited super-sensitivity to morphine, the neurons in the central gray displayed tolerance, and in some instances, dependence was evident when naloxone was administered. 4. The distribution of spontaneously active neurons within these two brain areas was found to be different in morphine-naive and morphine-dependent rats. 5. The electrophysiological findings of this study support the hypothesis of multiple opiate receptors.