The morphology of chromatin in spontaneously ovulated rat's oocytes was investigated. The degree of chromatin condensation was shown to vary widely: along with the typical cytological picture of metaphase II, both highly condensed (clamped) and decondensed ("not typical") forms of chromosomes were observed. A direct relation was discovered between the prevalence of the not typical morphological forms of oocyte chromosomes and the level of embryonal death. The shape of clamped and decondensed forms increased in the oocytes from old rats and in post-ovulatory aged oocytes.