Chronic ethanol increases the potency of certain delta opioid receptor (DOR)-selective agonists for thermal antinociception. Naïve C57BL/6 mice (n = 8–10) or mice (n = 8–9) that had chronically self-administered ethanol (see Methods and Materials) were injected intrathecally with increasing doses of a DOR-selective (deltorphin II [A], [D-Pen2,D-Pen5]-Enkephalin [B], SNC80 [C], or mu opioid receptor-selective (DAMGO [D]) agonist and thermal antinociception was measured using a radiant heat tail-flick assay. Data are represented as the percentage maximal possible effect, which is defined as [(measurement – baseline)/(cutoff – baseline)]*100. DPDPE, [D-Pen2,D-Pen5]-Enkephalin; MPE, maximal possible effect.