Partial ovaries from mice, hamsters, rabbits, Japanese monkeys and rats have survived deep-freezing and returned to a normal morphological state after being thawed and transplanted into the rat uterine cavity. This report describes the ice-free cryopreservation of mouse and other ovaries at -196°C by vitrification. The vitrification solution was based on the solutions reported by Rall & Fahy [16]. After ovaries had been exposed to the vitrification solution, they were frozen, with their suspending medium, by liquid nitrogen. After freezing, the ovaries were thawed in 37°C water. The viability of the previously frozen ovarian tissue was tested by transplanting it into the uterine cavity of pseudopregnant rats. Seven days after transplantation, the ovaries were removed with the rat uterus, and stained with haematoxylin and eosin for histological examination. Survival of the frozen-thawed the ovaries in the rat uterine cavity demonstrates that these ovaries can tolerate exposure to osmotic dehydration and vitrification in a concentrated solution of cryoprotectant and are then immunologically acceptable to the uterine cavity.