This article investigates the transient thermal field that occurs in the squirrel cage during motor start-up. First, the rotor currents occurring during start-up and the corresponding Joule losses are determined. Then the temperature distribution is calculated in the critical points of the rotor, instant by instant, by the finite element method. The study has been carried out on a motor whose shaft is connected to an operating machine having a remarkable gyratory moment, and the finite element analysis under transient state conditions has been made by means of a package entirely produced at the Department of Electrotechnics and Electronics of the Polytechnic of Bari.