The levels of immunoreactive Leu-enkephalin (ir-Leu-Enk) were measured in acid extract of hearts and lungs of adult normotensive Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Heart contents in ir-Leu-Enk were significantly higher in SHR rats (2.67 pmol/g) than in WKY rats (1.5 pmol/g). However, no significant change was observed in lung ir-Leu-Enk levels of these same animals (0.7-0.91 pmol/g). Reverse-phase high-performance liquid chromatography (RP-HPLC) of heart extracts indicated that the major form of ir-Leu-Enk in SHR rats coelutes with synthetic Leu-Enk and is increased by an approximate factor of 4.2-fold as compared with its eluting counterpart in the WKY chromatogram. The delta-selective opioid ligand, [3H][D-Pen2, D-Pen5]-Enk [( 3H]DPDPE) bound to two distinct receptor sites in membrane preparations of hearts from WKY rats with Kd of 1.33 and 22.8 nM and Bmax of 3.9 and 14.6 pmol/g protein, respectively. The binding with the heart membrane preparation of SHR rats displayed only the high affinity delta-binding site with a Kd of 1.53 nM and a Bmax of 6.5 pmol/g protein. These data indicate that the hypertensive state in SHR rats is manifested through a rise in the cardiac level of Leu-Enk and a selective down-regulation of its low-affinity delta-opioid receptor.