Pregnant CBA-mice were injected with different doses of "90Sr" on the 19th day post coitum. The doses ranged between 11.1 and 370 kBq per animal. The ovaries of the females treated in utero were analysed when the animals were 28, 56 or 84 days old, respectively. The effect of the nuclide was expressed as a reduction of the number (frequency) of cells in different stages of ovarian development in a comparison between "90Sr" treated animals and untreated controls. The injury (reduced number of cells) was strongly correlated to log dose. The most radiation sensitive stage (the naked oocytes) was clearly affected even at the lowest dose level. The different ovarian "90Y/90Sr" ratios in litters and their mothers are discussed.