We have developed a quantitative computerized subtraction technique to demonstrate in rat brain the regional distribution of mu1 sites, a common very-high-affinity binding site for both morphine and the enkephalins. Low concentrations of [D-Ala2, D-Leu5]enkephalin selectively inhibit the mu1 binding of [3H]dihydromorphine, leaving mu2 sites, while low morphine concentrations eliminate the mu1 binding of [3H][D-Ala2, D-Leu5]enkephalin, leaving delta sites. Thus, quantitative differences between images of sections incubated in the presence and absence of these low concentrations of unlabeled opioid represent mu1 binding sites. The regional distributions of mu1 sites labeled with [3H]dihydromorphine were quite similar to those determined by using [3H][D-Ala2, D-Leu5]enkephalin. High levels of mu1 binding were observed in the periaqueductal gray, medial thalamus, and median raphe, consistent with the previously described role of mu1 sites in analgesia. Other regions with high levels of mu1 binding include the nucleus accumbens, the clusters and subcallosal streak of the striatum, hypothalamus, medial habenula, and the medial septum/diagonal band region. The proportion of total specific binding corresponding to mu1 sites varied among the regions, ranging from 14% to 75% for [3H][D-Ala2, D-Leu5]enkephalin and 20% to 52% for [3H]dihydromorphine.