Xenpus laevis oocytes over a size range of 0.15--0.78 mm3 were dissected from their follicles and cultured in a defined medium for up to 28 days. Oocytes grew at average rates of 0.021 mm3.day-1 in the absence of insulin and 0.030 mm3.day-1 in the presence of insulin. The latter average growth rate corresponds to the fastest growth rate reported to date for oocytes in vivo. Oocytes grown in vitro can reach a size of at least 1.43 mm3, which is larger than the maximum size generally found in vivo. During growth in vitro; oocytes also acquire both a normal pigment pattern and, once they reach about 0.7 mm3, the ability to undergo complete maturation as a response to externally applied progesterone. These results show that Xenopus oocytes freed of their follicular investments are able to grow and differentiate in vitro.