In the rabbit muscles-antagonists in situ, the dependence of work (A) of isotonic contraction of the load (P) was studied in different age periods. For the range of loads 1.5 - Popt greater than or equal to P greater than or equal to 0.5 - Popt, the results were found to be satisfactorily approximated by the equation A = A opt = a - /Popt - P/2, where Aopt, Popt, and a are constants. The experiments revealed that the ratio Aopt. m. gastrocnemius/Aopt. m. tibialis anterior sharply increases during realization of the standing posture (16-18 days) and then drops at the grown-up age. Similar age changes were revealed for the Popt parameters ratio of the same muscles. At the same time, the respective ratio of the a parameters of the muscles under study drops sharply by the 16-18th day age, practically remaining the same afterwards. The obtained changes of the strength parameters and working ability of skeletal muscles in the course of postnatal ontogenesis are regarded from the standpoint of peculiarities of the skeletal muscles functioning at different age periods and with regard to ecological peculiarities of individual development of rabbits.