Uterine fluid (UF) was obtained from 12 women at midcycle and in the midluteal phase using a standardized sampling technique allowing comparison of samples by volume. Much greater volumes were encountered at midcycle (83-180 microL) than in the midluteal phase (5-35 microL). Six of the women who had previously been sterilized by laparoscopic coagulation of the fallopian tubes had lower midcycle volumes (83-127 microL) than had the women in the nonsterilized group (113-180 microL), indicating that UF is probably supplemented by fluid from the tubes at midcycle. Support for the theory that at least some of this additional fluid may be of peritoneal origin comes from observations on the concentrations of estradiol and progesterone in the peritoneal, tubal and uterine fluids.