Uterine cervical fibroblasts prepared from rabbits at 23 days of gestation were found to produce spontaneously the neutrophil chemotactic factor/interleukin 8 (IL-8). When the cells were treated with recombinant human interleukin 1 alpha and 1 beta (rhIL-1 alpha and -1 beta), both cytokines similarly enhanced the production of IL-8 in a dose-dependent manner. Recombinant tumour necrosis factor alpha also enhanced its production to a lesser extent, but interleukin 6 failed to modulate the production. Physiological concentrations of progesterone suppressed both the spontaneous and IL-1-mediated production of IL-8 in parallel with the decrease in the steady-state levels of its mRNA. These suppressive actions of progesterone were offset by co-treatment of cells with a progesterone antagonist, mifepristone (RU486). In conclusion, basal and IL-1-induced IL-8 production in rabbit uterine cervical fibroblasts is down-regulated by progesterone at the transcriptional level. These results obtained in vitro and our previous observations indicating that progesterone modulates the extra-cellular matrix breakdown via the suppression of production of matrix metalloproteinases and the augmentation of production matrix metalloproteinases and the augmentation of production of their specific inhibitors (TIMP-1) [Sato, Ito, Mori, Yamashita, Hayakawa and Nagase (1991) Biochem. J. 275, 645-650] may explain the mechanisms of the maintenance of pregnancy until parturition and the acceleration of uterine cervical ripening and dilatation at term.