We used the method of reverse transcription-polymerase chain reaction (RT-PCR) to observe the effects of long-term administration of peptide delta opioid receptor agonist DPDPE and non-peptide delta agonist BW373U86 on the mRNA expression of delta receptor and the possible difference between the two agonists. The results showed: (1) the level of delta receptor mRNA decreased significantly 24-48 h after administration of DPDPE; the effect of BW373U86 lasted only 24 h; and (2) DPDPE produced a significant decrease of delta-receptor mRNA at 10(-6) mol/L, whereas BW373U86 was effective only at 10(-5) mol/L. These results suggest that long-term administration of both peptide delta opioid receptor agonist (24, 48 h) and non-peptide delta opioid agonist (24 h) significantly decreased mRNA expression of delta opioid receptor and the effect of peptide agonist DPDPE seems to be stronger and last longer than that of non-peptide agonist BW373U86.