This study reports observations on fine-structural changes in the nucleus of the oocyte in hamsters during the establishment of the primordial (unilaminar) follicle at 7 to 12 days after birth. Following pachytene, the nuclei at early diplotene contain simple chromosomal cores surrounding by a sheath of fibrillo-granular material symmetrically disposed in the form of lateral projections and loops. From 9 days onwards increasing numbers of the oocytes show nuclei lacking such chromosomal threads: instead, they contain randomly disposed, condensed fibrillar clumps with associated dense knots of material, and prominent aggregations of 25--30-nm granules. This second nuclear form is assumed by all oocytes examined at 11 and 12 days and appears to be the definitive diplotene phase. These changes in the appearance of the nuclear chromatin are discussed in the light of those found in oocytes of other species, and it is concluded that the hamster diplotene phase consists of a transitory 'compace' or uniform lampbrush stage, followed by a definitive dictyate condition like that in the rat, but with markedly differential condensation or extreme diffusion of lateral loops. This conclusion is examined in relation to findings which report a marked increase in the sensitivity of hamster oocytes to X-radiation at the time that the observed changes in nuclear configuration are taking place. The combined observations support the hypothesis that the dictyate stage in rodent oocytes represents a modified diplotene phase in which the high degree of spatial diffusion of components of chromosomes of the lampbrush type results in a reduced capacity for repair following exposure to ionizing radiations.