Interactions between delta-opioid receptors and morphine-preferring mu-opioid receptor subtypes have been suggested. Availability of transgenic mu-opioid receptor knockout mice allows assessment of mu-opioid receptor roles in the analgesia produced by the classical delta-opioid receptor agonist [D-Pen2,D-Pen5]enkephalin (DPDPE) in hot-plate and tail-flick tests. DPDPE analgesia was dramatically reduced in mu-opioid receptor knockout mice in a gene-dose-dependent fashion. The analgesia induced by this classic delta-opioid receptor agonist depends on intact mu-opioid receptors, suggesting that selective delta-opioid receptor drugs may require mu-opioid receptor occupancies for full efficacy.