Deltorphins show a high affinity and selectivity for delta opioid receptors. Analogs of deltorphins with substitution of Val residues with more hydrophobic Ile appear to have a higher in vitro activity and selectivity than parent deltorphins. In our study, changes in the nociceptive threshold after intrathecally injected deltorphin I (DELT I), deltorphin II (DELT II) and their Ile - derivatives (ILE-DELT I and ILE-DELT II, respectively) were investigated in a tail-flick (TF) and a paw pressure (PP) tests. Male Wistars rats (260-350 g) with a chronically implanted catheter in the lumbar enlargement of the spinal cord were used. DELT I and DELT II, injected i.th. in doses of 0.15, 1.5 and 15 microg, increased the TF latency in a dose-dependent manner. The effect of their derivatives was similar, but the action of ILE-DELT II was shorter than that of the parent peptide. In the PP test, the antinociceptive effects of DELT I and their derivative ILE-DELT I were similar, but the effect of a higher dose of ILE-DELT I lasted longer in comparison with the parent peptide. Both DELT II and ILE-DELT II exhibited a low and short-lasting antinociceptive potency in the PP test. The effect of DELT I (1.5 microg) was antagonized by pretreatment with NTI (30 microg), a non-selective delta opioid receptor antagonist, as well as by the delta2 receptor antagonist NTB (3 microg) and the delta1 antagonist BNTX (1 microg) in both those tests used. The antinociceptive effect of DELT II (1.5 microg) was antagonized by pretreatment with NTI (30 microg) and NTB (3 microg) in the TF test, but not in the PP test. In the latter test, the antinociceptive effect of DELT II was potentiated by pretreatment with BNTX (1 microg). The effects of both the derivatives ILE-DELT I and ILE-DELT II were antagonized by NTI (30 microg) in the TF test, and by NTI (30 microg) and NTB (3 microg) in the PP test. Like in the case of the parent peptide, the effect of ILE-DELT II was potentiated by pretreatment with the delta1 antagonist BNTX (1 microg). Summing up, modification of the DELT I and II by substituting Ile for Val residues appears to influence the delta selectivity rather then the potency of the peptides at spinal delta receptors.