In addition to producing antinociception, opioidsexert profound effects on body temperature. This studyaimed at comparing antinociceptive and hyperthermicresponses between two groups of l-opioid receptor agonists:fentanyl (4-anilinopiperidine-type) and morphine(phenanthrene-type) derivatives in rats. Analgesic activitywas assessed by tail immersion test and the body temperatureby insertion of a thermometer probe into the colon. Fentanyl (F), (±)-cis-3-methyl fentanyl (CM), (±)-cis-3-carbomethoxy fentanyl (C), (±)trans-3-carbomethoxyfentanyl (T) and (±)-cis-3 butyl fentanyl (B) produceddose-dependent increase in antinociception and hyperthermia. The relative order of analgesic potency was:CM(11.27)[F(1)[C(0.35)CT(0.11)CB(0.056). Similar tothis, the relative order of hyperthermic potency was:CM(8.43)[F(1)[C(0.46)CT(0.11)CB(0.076). Morphine(M), oxycodone (O), thebacon (T) and 6,14-ethenomorphinan-7-methanol, 4,5-epoxy-6-fluoro-3-hydroxy-a,a,17-trimethyl-, (5a,7a) (E) also produced dose-dependentincrease in antinociception and hyperthermia. Amongmorphine derivatives the relative order of analgesicpotency was: E(56)[O(5)CT(2.6)[M(1), and similar tothis, the relative order of hyperthermic potency was:E(37)[O(3)CT(2.3)[M(1). Morphine (phenanthrene-type)and fentanyl (4-anilinopiperidine-type) derivatives producedhyperthermia in rats at doses about 2 times lower,and 6–11 times higher, than their median antinociceptivedoses, respectively. This study is first to identify differencebetween these two classes of opioid drugs in their potenciesin producing hyperthermia. Further studies are needed toclarify the significance of these findings.