The pattern of change in the levels of prolactin (PRL) in cervical mucus was studied in 19 normal ovulatory women. The concentration of PRL in cervical mucus was generally higher than that in plasma except at midcycle, when a reverse relationship was observed. The concentration of PRL in mucus obtained at midcycle was lower than mucus obtained during follicular phase or luteal phase. There were no significant differences in the total PRL content among preovulatory, midcycle and postovulatory cervical mucus samples. The amount of cervical mucus markedly increased during the periovulatory period (two days from the peak of plasma luteinizing hormone). The cervical mucus PRL level undergoes cyclic changes during the normal menstrual cycle, and the decrease in PRL concentration in mucus at midcycle is most likely a result of the dilutional effect of increased mucus volume. The physiological implication of the presence of PRL in human cervical mucus is also discussed.