Right and left guinea-pig atria responded to decreasing temperatures (42-27 degrees C) with elevation for force of contraction and concomitant increases in cAMP. When atria were rapidly cooled from 42 to 27 degrees C the increase in cAMP occurred prior to the onset of the inotropic responses. Papaverine (3 X 10(-5) M) potentiated the effects of temperature on cAMP and force of contraction on left atria driven at 0.5 Hz. On right atria beating spontaneously at frequencies above 2 Hz papaverine only potentiated the effect of decreasing temperatures on the response of cAMP but not on that of force of contraction. Time course studies of the effects of isoprenaline (3 X 10(-8) M) on right atria at 27 degrees C showed large inotropic responses to isoprenaline which were accompanied by increases in cAMP. At 42 degrees C the responses of force of contraction and cAMP to isoprenaline occurred faster and were only short-lasting. As with the time courses for isoprenaline, dose-response curves for the effect of isoprenaline and papaverine on cAMP content and force of contraction also appeared to be shifted towards higher levels at hypothemia. However, pD2 values reflected increases in affinity for inotropic, but not for the cAMP responses to isoprenaline and papaverine at hypothermia. These results show that cyclic AMP is involved in the inotropic responses to hypothermia, but not in the supersensitivity of heart to isoprenaline and papaverine as observed at low temperatures.