[en] Total radiative power losses were measured with a pyroelectric detector on the CASTOR tokamak in a broad range of plasma parameters. It was shown that while the most important channel of energy losses for the low density operation (average n_e< or approx.=10¹⁹ m^-3) is thermal conductivity, the high density regimes are radiative dominant. Using a simple analytic energy balance model, the connection is discussed between such a high level of radiation and the shrinking of the current channel resulting in an enhanced MHD activity. (author)