This study searched for polymorphic sites in the murine mu-, delta- and kappa-opioid receptors that presumably influence pain perception. We employed two mouse lines divergently bred for high (HA, high analgesia line) and low (LA, low analgesia line) swim stress-induced analgesia (SSIA). These mouse lines differ substantially in pain sensitivity, measured as hind paw withdrawal latency in the hot-plate test. We found a novel C320T transition in exon 2 of the delta-opioid receptor gene, resulting in an A107V substitution in the first extracellular loop (EL1) of the peptide chain. Using hot-plate and tail-flick tests, we found a significant association between the genotype of this locus and basal nociception and SSIA magnitude. Moreover, this transition affects the pharmacological effects of two specific delta-opioid receptor ligands, the agonist SNC80 and the antagonist naltrindole. The impact of the C320T polymorphism on the magnitude of SSIA was partially mediated by endogenous opioids. The effectiveness of exogenous delta-opioid receptor ligands was greater in the HA than in the LA line, and was greater in C320C homozygotes than in C320T heterozygotes within each line. Our results indicate that the C320T polymorphism in the delta-opioid receptor gene is at least partly responsible for the divergent nociceptive thresholds in HA and LA mice under both basal and post-stress conditions.