SUMMARY: Follicle-stimulating hormone (FSH) labelled with 125I which retained its biological activity was injected into immature mice. Accumulation in, and loss from, the ovaries and some other tissues were studied. The ovarian content was maximal 40 min after injection. By 30 min the specific radioactivity of the ovary was proportional to that of the blood. Previous injection of a large dose of unlabelled FSH reduced accumulation of 125I-labelled FSH by one-third only. Subsequent injection of a large dose of unlabelled FSH accelerated loss of 125I-labelled FSH. It was concluded (i) that the 125I-labelled FSH behaved like normal FSH; (ii) that the immature mouse ovary has numerous binding sites; (iii) that the attachment of FSH to the binding sites is labile; (iv) that the concentration in the ovary tends to equilibrate with that in the blood. Similar conclusions were drawn from parallel studies using cultured infantile mouse ovaries. These also suggested that depletion of 125I-labelled FSH from the ovary involved a second-order reaction.