Rat uterine fluid Na+ and K+ concentrations were calculated from the ratio of the Na+ and K+ concentrations in uterine washings, on the assumption that the sum of Na+ and K+ (mequiv./l) is the same in blood serum and in uterine fluid. Uterine fluid volumes were likewise calculated from the total quantity of Na+ and K+ washed out of the uterine horn.The mean uterine fluid volume was 7\m=.\6 \ g=m\ lper horn and the mean potassium concentration was 45\m=.\8 mequiv./l on day 6 of normal pregnancy. Both oophorectomy alone on day 2 and oophorectomy followed by daily injections of progesterone, cause a marked reduction of the uterine fluid volume on day 6 of pregnancy. Oophorectomy on day 2 caused a significant reduction in the uterine fluid K+ concentration. Administration of high and low doses of progesterone after oophorectomy on day 2 of pregnancy resulted in uterine fluid potassium concentrations higher and lower, respectively, than the normal day 6 fluid.It is postulated that a high uterine fluid K+ concentration is essential for blastocyst-endometrial contact preceding implantation and that this may be significant with regard to certain types of delayed implantation.

Summary Human uterine fluid was obtained from women at laparotomy by a new technique which avoids contamination by cervical and tuba1 fluids. The potassium (K+) concentration of the uterine fluid rose from 17.9 mEq./l. in the follicular phase to 33.7 mEq./l. in the luteal phase (p<0.00l). However, there was a decreased uterine fluid volume in the luteal phase and the total potassium content of the fluid was not increased, suggesting reabsorption of sodium and water rather than the secretion of more potassium. The high uterine fluid K+ concentration, which may reach 39 mEq./l. in the mid‐luteal phase, presumably provides a suitable medium for the blastocyst; there are also reasons for believing that the high K+ concentration of the luminal fluid will reduce the membrane potential of the endometrium, that this will facilitate blastocyst‐endometrial contact and may be essential for blastocyst‐endometrial adhesion.