The delta opioid ligands, [3H]DPDPE (delta 1) and [3H]DSLET (delta 2) were used in quantitative autoradiographic experiments to ascertain whether separate populations of delta opioid subtypes could be identified in rat brain. Densitometric image analysis showed a general similarity in delta 1 and delta 2 distributions. However, statistically significant differences in binding levels were observed in anatomically discrete regions. Examples of these regions and their delta 2/delta 1 ratio(s) are: dorsomedial hypothalamus (9.3), ventromedial hypothalamus (4.9), superior colliculis (2.7), medial division of bed nucleus stria terminalis (1.6-3.0), external cortex of the inferior colliculis (2.1), amygdaloid nuclei (1.5-2.1), cingulate cortex (1.8), CA1, CA2, and CA3 regions of Ammon's horn (1.6-2.0), dentate gyrus (1.7), laminar VI of the frontal, forelimb, hindlimb and parietal cortices (1.3-1.8), nucleus accumbens (1.4) and caudate/putamen (1.3). These findings provide evidence supporting the existence of distinct delta 1 and delta 2 opioid receptors.