Antiprogestins are used to induce first trimester abortion and to dilate the cervix before vacuum aspiration. Cervical dilatation is associated with profound changes in the connective tissue. In what respect antiprogestins interfere with this process has hitherto been sparsely investigated. The aim of present study was to examine the influence of the antiprogestin mifepristone on cervical collagen synthesis in nonpregnant, early and late pregnant women. The effects were compared with those of progesterone. The content of collagen in cervical tissue was determined by measuring hydroxyproline. Collagen synthesis was studied in vitro either by incubation of cervical tissue specimens from women, pretreated with mifepristone in vivo, in the presence of 14C-proline or by incubation of cervical tissue of not pretreated women in the presence of the isotope and mifepristone or progesterone. Pretreatment with mifepristone, but not progesterone, induced a significant increase in cervical dilatation. The cervical concentration of collagen was not altered after mifepristone administration. Pretreatment with mifepristone did not quantitatively influence the time course of radiolabeling in vitro or the pattern of radiolabeling in different protein components as revealed by electrophoresis. In vitro mifepristone, like progesterone, reduced the incorporation of 14C-proline. From the present data we conclude that mifepristone pretreatment in connection with first trimester abortion is not associated with any major changes, qualitatively or quantitatively, of collagen synthesis. However, we cannot exclude that mifepristone still may affect the de novo formation of collagen since mifepristone, administered in vitro, did reduce collagen synthesis.