Pretreatment of male ICR mice with an antisense oligodeoxynucleotide to delta-opioid receptor mRNA (DOR AS oligo, 163 pmol) given intrathecally (i.t.) once a day for 1-3 days produced a time-dependent attenuation of antinociception produced by i.t.-challenged [D-Ala2] deltorphin II (6.4 nmol), a delta-opioid receptor agonist. The attenuation of the [D-Ala2]deltorphin II-induced antinociception caused by pretreatment with DOR AS oligo given i.t. daily was blocked by co-pretreatment with naltriben (14.5 nmol), a delta-opioid receptor antagonist, but was markedly enhanced by concomitant pretreatment with thiorphan (19.7 nmol) or bestatin (14.5 nmol), which inhibits the degradation of endogenously released Met-enkephalin. Concomitant pretreatment with antiserum to Met-enkephalin, but not with antiserum to Leu-enkephalin, beta-endorphin or dynorphin A (1-17), and DOR AS oligo given i.t. daily for 3 days prevented the attenuation of i.t.-challenged [D-Ala2]deltorphin II-induced antinociception caused by the DOR AS oligo pretreatment. Our results support the existence of a turnover of delta-opioid receptors in the mouse spinal cord caused by the release of Met-enkephalin.