We developed a calorimetric technique to measure the isothermal magnetocaloric entropy change. The method consists in the use of Peltier cells as heat flow sensor and heat pump at the same time. In this paper, we describe the setup, the constitutive equations of the Peltier cell as sensor and actuator, and the calibration procedure. The Peltier heat is used to keep the sample isothermal when magnetic field is changed. The temperature difference between the sample and the thermal reservoir is kept by a digital control within 5 mK for a magnetic field rate of 20 mT s(-1). The heat flux sensitivity around 1 microW. With this method, it is possible to measure the magnetocaloric effect in magnetic materials by tracing the curves of the exchanged entropy Delta(e)s as a function of the magnetic field H. The method proves to be, in particular, suitable to reveal the role of the entropy production Delta(i)s, which is connected with hysteresis. Measurement examples are shown for Gd, BaFe(12)O(19) ferrite, and Gd-Si-Ge.