Intrathecal (i.t.) pretreatment of male ICR mice with antisense oligodeoxynucleotide to delta-opioid receptor mRNA once a day for 1-3 days caused a time-dependent attenuation of i.t. administered [D-Ala2]deltorphin II-induced antinociception as measured by the tail-flick test. The attenuation of the antinociception induced by i.t. administered [D-Ala2]deltorphin II, a delta-opioid receptor agonist, was enhanced by i.t. pretreatment for 1 day with [D-Ala2]deltorphin II, but not [D-Ala2,N MePhe4,Gly(ol)5]enkephalin (DAMGO), a mu-opioid receptor agonist, or U50,488H, a kappa-opioid receptor agonist, given together with antisense oligodeoxynucleotide to delta-opioid receptor mRNA. The present results indicate that stimulation of spinal delta-opioid receptors by i.t. injection of [D-Ala2]deltorphin II selectively causes a loss of delta-opioid receptor-mediated antinociception in mice pretreated with antisense oligodeoxynucleotide to delta-opioid receptor mRNA.