Previous studies have investigated the dynamic stability of natural circulating flows inside closed loops. The systems considered have an external heat source, and the circulating fluids do not generate heat internally. In this work, the effects of internal heat generation on the stability of natural circulating flows are investigated for the first time. A semianalytical linear method and a numerical nonlinear method are applied in order to characterize the effects of internal heat generation on the stability maps of different closed loop configurations. Results show that when internal heat generation dominates the external heat source, it can modify the shape and area of the stability regions. Among the different loop configurations studied, the Vertical Heater-Vertical Cooler (VHVC) configuration shows a better stability behavior.