The meiotic behaviour in vitro of the ova from a polyovular strain and a polycystic strain was normal up to the 2nd metaphase-1st polar body stage at 21-200 days of age. No numerical or structural abnormality was detected. The av. chiasmata frequencies/bivalent at 1st metaphase ranged from 1.21 to 1.23 in the polycystic strain, and from 1.23 to 1.32 in the polyovular strain. In the latter group, the mono-ovular follicles were similar to the polyovular follicles. The data suggest that the relative infertility of these strains is not due to a gross chromosome abnormality in the ova.