Dictyate oocytes liberated from the ovaries of sexually mature mice can resume meiosis when cultured in vitro (Donahue, 1968). The present study was designed to determine the postnatal age when oocytes acquire this capacity. Female Bagg mice from a closed colony were killed by decapitation or cervical dislocation at age 11, 13, 15 or 17 days. One ovary from each animal was fixed in Bouin solution and stained with Mayer's haematoxylin and eosin (control ovary). The other ovary was incubated at 37 °C in phosphate-buffered saline for 20 min and in 0·1% trypsin (Difco 1:250) in phosphate-buffered saline for 30 min. The ovarian tissue was then rinsed in phosphate-buffered saline and dispersed by vigorous sucking with a pipette in a fresh change of buffered saline. This was performed under visual control in a dissecting microscope. Liberated oocytes were immediately transferred to culture medium with a micropipette. The medium and the