Short-lasting decrease in rectal temperature in mice after intraperitoneal administration of an enkephalin dimer, the so called double-enkephalin--(Tyr-D-Ala-Gly-Phe-NH)2 (D-ENK-O)--at dose 0.1, 0.5, 1, 2.5, 5, 10 and 20 mg/kg of body weight was observed. Another double-enkephalin--Tyr-D-Ala-Gly-Phe-NH-(CH2)3-HN-Phe-Gly-D-Ala-Tyr-- fai led to produce this effect. The hypothermic effect of D-ENK-O was almost completely reduced by naloxone which suggests an involvement of opiate receptors in the D-ENK-O produced hypothermia in mice.