The relationship between the ash content of rabbits' metatarsals and their modulus of elasticity, static strength, work to failure and impact strength was determined. Over the range of ash contents observed the static strength and the modulus of elasticity increased linearly with the ash content. The energy that could be absorbed by the bone material, in both static and dynamic loading, increased and then decreased with ash content. The highest values for energy absorption were roughly at the median value of ash. It is proposed that this value is the optimum value in life, because bones are usually broken by impact, and energy absorption is important in resisting impact. A short discussion of the reason for the rise and fall in energy absorption is given.