1 Binding assays on homogenates of guinea-pig brain showed that the maximal number of binding sites was different tritiated ligands interacting with the opiate receptors. 2 At 25 degrees C the binding capacity of morphine or dihydromorphine was only about 3 pmol/g fresh brain whereas etorphine and D-Ala2-L-Leu5- and D-Ala2-L-Met5-enkephalin amide had capacities of 13 to 15 pmol/g brain. D-Ala2-D-Leu5-enkephalin had an intermediate capacity of about 6 pmol/g brain. 3 The binding capacities of the natural methionine- and leucine-enkephalins measured at 0 degrees C were 5 to 6 pmol/g brain. At this temperature, the binding capacity of dihydromorphine, D-Ala2-D-Leu5-enkephalin and of the two enkephalin amides was only slightly lower than at 25 degrees C. 4 In assays in which unlabelled ligand competed with the same labelled ligand, the inhibition constants (KI) were equal to or not more than twice as large as the equilibrium dissociation constant (KD) determined in saturation assays. In contrast, the KI of unlabelled dihydromorphine against [3H]-D-Ala2-D-Leu5-enkephalin or of unlabelled D-Ala2-D-Leu5-enkephalin against [3H]-dihydromorphine was about 20 times higher than the respective KD values. 5 When for a given compound the ratio of the KI value against [3H]-D-Ala2-D-Leu5-enkephalin to the KI value against [3H]-dihydromorphine (discrimination ratio) is calculated, a high value indicates selectivity in favour of the mu-receptor and a low value selectivity in favour of the delta-receptor. The most selective mu-agonist known so far is normorphine with a discrimination ratio of 70 and the most selective delta-agonist is D-Ala2-D-Leu5-enkephalin with a ratio of 0.11. The selectivity of the known antagonists is in favour of the mu-receptor, since their discrimination ratios are larger than 1, varying between 10 for naloxone and 4 for Mr 2266.