A simple method is described for obtaining repeated measurements of body temperature in young mice with minimal error introduced through stress. Temperatures are measured by an external thermocouple attached to the thorax in the region of the heart. A sling around the animal's thorax provides insulation for the thermocouple from surrounding air and mild restraint of the animal during measurement. Comparison with temperatures obtained in other locations indicates that external thoracic temperature as described gives a reliable estimate of true body temperature in mice up to 21 days old. Typical results with normal mice aged 1 day to 6 wk postpartum are included, together with a brief discussion of the technique's applicability.