The heat capacity C P ͑T͒ of the ferromagnetic compounds RAl 2 ͑R =Pr,Nd,Tb,Dy,Ho,Er͒ was measured at zero and applied magnetic field of 5 T in the temperature interval from 2 to 200 K. From these results are calculated the magnetic component of the entropy change, −⌬S mag ͑T͒ = S͑0,T͒ − S͑H , T͒. From resistivity measurements, ͑H , T͒, from 2 to 300 K in the same compounds, we calculated the resistivity change due to the applied magnetic field, −⌬ mag ͑T͒ = ͓ mag ͑0,T͒ − mag ͑H , T͔͒. The results are compared and we observed a similar dependence between −⌬ mag ͑T͒ and ͑T / T C ͒ m ⌬S mag ͑T͒ with m = 0 for T ജ T C and m = 1 for T ഛ T C. A simple model using a Hamiltonian considering molecular and crystalline electric fields, in a mean field approximation, is adopted for the calculus. Our results show that theory and experiment are in good agreement showing that the magnetoresistivity is a probe to the field-induced change of magnetic entropy in these compounds and can be extended to other materials. A model for the factor connecting both quantities, −⌬S mag ͑T͒ and −⌬ mag ͑T͒, is developed. This factor contains mainly the effective exchange integral which is related to Fermi energy that in turn is related to the electron effective mass.