The effects of temperature on spontaneous action potential activity in the 7-9 somite embryonic precontractile chick hearts were investigated using an optical method for monitoring membrane potential. A potential-sensitive merocyanine-rhodanine dye was used as an optical probe. Cooling caused a marked decrease in the frequency of occurrence of the spontaneous action potential. This slowing was associated with a decreased rate of diastolic depolarization of the pacemaker action potential. When the temperature was lowered, a decrease in the rate of rise and a prolongation of duration of the optical action signal also occurre. In addition, cooling resulted in a disordering of the rhythmicity of the action potential, and a decrease in the conduction velocity of excitation. Q10 values for such parameters were calculated: there were slight decreases in the Q10 values for the frequency and conduction velocity for 7-9 somite embryonic hearts.