The present invention is related to pyrrolidine oxadiazole and thiadiazole derivatives for use as pharmaceutically active compounds, as well as pharmaceutical formulations containing such pyrrolidine oxadiazole derivatives. Said pyrrolidine derivatives are useful in the treatment and/or prevention of preterm labor, premature birth and dysmenorrhea. In particular, the present invention is related to pyrrolidine derivatives displaying a substantial modulatory, notably an antagonist activity of the oxytocin receptor. More preferably, said compounds are useful in the treatment and/or prevention of disease states mediated by oxytocin, including preterm labor, premature birth and dysmenorrhea. The present invention is furthermore related to novel pyrrolidine derivatives as well as to methods of their preparation. (I) B is a oxadiazole or thiadiazole group ##STR00001##