AbstractAnalysis of age changes in physical characteristics of mouse limb bones requires a sensitive, consistent and relatively simple apparatus for determination of transverse breaking strength. The developed instrument is essentially a one meter counter‐balanced 4‐to‐1 lever with a tracked, motor‐driven weight on its upper surface. At the beam's short end a loading tool impinges upward on a bone mounted in an adjacent holder; at the opposite end is a mercury switch connected to a relay halting the weight motor when fracture occurs. Bones, with proximal ends embedded in convex securing blocks of impression medium, may be positioned in a standard relationship to the holder, loading tool edge, and the balanced beam.Repetitive loading of a steel spring indicates instrument error of 1.8%. Preliminary tests with a random series of paired mouse femora show no significant difference between left and right bones. In the long‐term study it will be possible to correlate physical strength with other characteristics of opposite bones.An air‐gap capacitor across loading tool and bone holder and coupled to a variable frequency oscillator permits monitoring of specimen elasticity on a strip‐c