The photothermal properties and heat diffusion of polymeric lasers, made up from solutions of Rhodamine 6G in solid matrices of poly(2-hydroxyethyl methacrylate) with different amounts of the cross-linking monomer ethylene glycol dimethacrylate and copolymers of 2-hydroxyethyl methacrylate and methyl methacrylate have been studied through photothermal deflection spectroscopy. The heat load that is due to the pumping process was quantified as a function of the pump excitation repetition frequency (0.25-10 Hz), determining the time-dependent temperature changes at different locations within the laser matrix. A theoretical model, which reproduces these changes with high accuracy, was developed on the basis of the heat-diffusion equation of optically dense fluids. The observed thermal effects became important for impairing the laser stability at pump repetition frequencies higher than 1 Hz. In addition, the irreversible optical changes produced in the laser matrices at high pump fluence values (>1 J/cm2) were also analyzed. These effects originate, most likely, from a two-step photothermal mechanism.