The aim of the present study was to examine whether changes in temperature alter the effects of morphine on isolated right atria. Morphine dose-response curves at different temperatures (30, 37 or 40 degrees C) from right atria of the rat were obtained. The maximal negative chronotropic effect induced by morphine was 11 +/- 3% at 37 degrees C and 20 +/- 1% at 30 degrees C. However, when the temperature was increased to 40 degrees C, morphine induced a positive chronotropic effect. The increase in auricular automaticity induced by morphine at 40 degrees C was antagonized in the presence of naloxone (5 x 10(-6) mol/l). These data demonstrated that: (1) hyperthermia modifies the effects of morphine on isolated right atria and (2) at 40 degrees C morphine induces a positive chronotropic effect by activation of opioid receptors.