The pH, protein concentration, and amount of oestrous uterine fluid were determined in mature Dutch type rabbits from 1 to 20 weeks after ligation of the uteri. Uterine fluid accumulated at the rate of 2·10 to 4·48 ml per cornu per week. These fluids were serous, slightly turbid, and colourless or slightly yellow. The protein concentration decreased from 5·13 mg/ml after the first week to 1·82 mg/ml after the 20th week of accumulation. The high initial protein concentration may have been due to contamination of the uterine fluid with blood at the time the uterus was ligated. The in-utero pH of fluids was 7·64. A rapid increase in pH with time and handling after removal from the uterus indicated the importance of carbon dioxide in buffering uterine fluids.