We have computed the perturbation to the infrared radiative heating rates of the lower stratosphere due to the occurrence of polar stratospheric clouds (PSCs) during the winter season in the Antarctic and Arctic regions. The calculations were made with a multispectral radiative transfer code that allows for scattering, absorption, and thermal emission by particles and gases. We investigated perturbations due to both particulate opacity of the PSCs (direct effect), and to the partial condensation, and hence, decrease of H20 vapor accompanying their formation (indirect effect). For plausible values of model parameters, the direct effect is always one of increased radiative cooling, while the indirect effect is always one of decreased cooling. On a synoptic time scale of a single PSC event (∼ days), the net effect is probably one of enhanced cooling, with its magnitude having an important impact on stratospheric heating rates only for the most optically thick PSCs (extinction coefficient > 10−1 km−1... Abstract We have computed the perturbation to the infrared radiative heating rates of the lower stratosphere due to the occurrence of polar stratospheric clouds (PSCs) during the winter season in the Antarctic and Arctic regions. The calculations were made with a multispectral radiative transfer code that allows for scattering, absorption, and thermal emission by particles and gases. We investigated perturbations due to both particulate opacity of the PSCs (direct effect), and to the partial condensation, and hence, decrease of H20 vapor accompanying their formation (indirect effect). For plausible values of model parameters, the direct effect is always one of increased radiative cooling, while the indirect effect is always one of decreased cooling. On a synoptic time scale of a single PSC event (∼ days), the net effect is probably one of enhanced cooling, with its magnitude having an important impact on stratospheric heating rates only for the most optically thick PSCs (extinction coefficient > 10−1 km−1...