Summary The detailed mechanisms which can explain the inherent radiosensitivity of salivary glands remain to be elucidated. Although DNA is the most plausible critical target for the lethal effects of irradiation, interactions with other constituents, such as cell membrane and neuropeptides, have been suggested to cause important physiological changes. Moreover, mast cells seem to be closely linked to radiation-induced pneumonitis. Therefore, in the present study the effects of fractionated irradiation on salivary glands have been assessed with special regard to the appearance of mast cells and its correlation with damage to gland parenchyma. Sprague-Dawley strain rats were unilaterally irradiated to the head and neck with the salivary glands within the radiation field. Although the detailed mechanisms by which radiation induces damage in biological tissues are not fully understood, it is generally accepted that generation of free radicals is of major importance. DNA is the most likely target for the lethal effects of these irradiation induced free radicals, however it is plausible that other compartments, i.e. non-genetic macromolecules, such as proteins and carbohydrates, can be significantly affected with vital alterations in the functional integrity of the certain biological tissues (Creasy, 1960;Desai et al., 1964;Sutherland et al., 1967;Alper, 1977;El-Mofty et al., 1981;Franzen et al., 1991;1993). In addition, changes caused by external factors and in the surrounding tissues can play an important role in modifying the final detectable and evaluable reaction, which indeed has recently been proposed, e.g. ongoing smoking depressed pneumonitis caused by irradiation in humans (Franzen et al., 1989;Bjermer et al., 1990; as well as in experimental animals . Moreover, sublethal damage following fractionated irradiation seems to cause alterations in cell membrane function and in physiological parameters (Reade & Steidler, 1985; Stephens et al., 1986a,b;Franzen et al., 1991). Most recently, it was also demonstrated that irradiation enhances the expression of important neutopeptides/growth factors, at least in salivary glands (Forsgren et al., 1992;Franzen et al., 1993). Furthermore, parotid acinar cell death following irradiation has been described as an interphase death (Stephens et al., 1986a,b;Franzen et al., 1991).The remarkable sensitivity of salivary gland acinar cells compared with other glands to radiation is a unique radiobiological phenomenon (Shannon et al., 1978;Mira et al., 1982;Abok et al., 1984;Junglee et al., 1986;Franzen et al., 1991). Other well-differentiated gland cells are known to be more or less radioresistant (Rubin & Casarett, 1972). The exact mechanism which explains this sensitivity is not entirely understood. Since mast cells have been proposed to interact in a substantial manner with the development of radiationinduced pneumonitis and pulmonary fibrosis (Franzen et al., 1989;Bjermer et al., 1990; Nilsson et al., 1990a,b; we found it of interest to evaluate the correlation of connecti...