1. Potassium movements have been studied in vitro in uteri of estrogenpretreated rats with42K as a tracer. At 37 °C the uterus was nearly in potassium balance in Krebs–Ringer bicarbonate and the exchange of potassium was adequately described by a single exchange constant, aside from a small fast fraction (17%) which probably contains potassium located superficially on cells as well as the extracellular potassium. No difference could be detected in their rates of exchange of potassium between two portions of the uterine horn, one containing only the longitudinal muscle layer and the other containing the remainder of the wall. The potassium exchange before or after flux correction for diffusion delay was about 5 or 9 moles cm−2sec−1, using a value of v/a of 1.8. There was a slow gain of sodium and water unrelated to potassium loss, attributed to expansion of the extracellular fluid.2. When the temperature of the Ringer fluid was reduced, the uterus remained in potassium balance at 27° and 17 °C. At 7 °C there was a net loss of potassium and exchange could no longer be described by one constant. On going from 37 to 7 °C the uterine horns shortened and the suggestion was made that muscle cells were depolarized initially by cold, or exuded water accounting for the rapidly exchanging fraction of potassium observed at this temperature. Loss of radioactive potassium from the myometrium owing to depolarization and associated with contraction appeared to account for the inhomogeneity on going to 7 °C. The Q10for influx of potassium between 27 and 7 °C was about 3 while that for efflux was about 1.6, excluding the fast fraction present at low temperatures. The Q10for efflux was diminished by depolarization and that for influx increased so that both may have been about 2. When uterine horns were stored overnight in the cold, they lost potassium and gained sodium, chloride, and water, but these ion changes were reversed on rewarming.