Summary and Conclusions.The lower and upper temperature limit for the spontaneous mechanical activity of the isolated auricle of hibernating (hedgehog and hamster) and non‐hibernating (rat and partly rabbit) mammals has been determined. Besides this, some observations have been made of the mechanogram of hedgehog and hamster auricle at various temperatures.It was found that:1) The isolated auricle of a hibernating mammal ceases to beat at a temperature (1.5° to 6° C) definitely lower than that of a non‐hibernating mammal (16° to 18° C on average).2) Irreversible heat standstill of a hamster and rat auricle was observed at about 46.6° C and 48.7° C respectively. This was preceded by irregularities in rhythm at about 2° C lower temperature. The hedgehog auricle stopped suddenly at about 37° to 40° C. This standstill was reversible.3) The rate of the hamster auricle was a linear logarithmic function of the temperature, contrary to the case with rabbit and rat auricles; this observation indicates species differences.4) The ratio duration of contraction to cycle length increased with rising temperature in the hedgehog auricle, which differs from the rabbit