AbstractDevelopment of a characteristic localized dysplasia of the proximal tibial epiphysis, occurring in high incidence in offspring of manganese‐deficient rats, was studied. Young of female rats, maintained from weaning on a diet deficient in manganese, were compared with those of females receiving the same diet with manganese supplementation. Epiphyses were studied histologically and in alizarin‐stained specimens. In offspring of deficient females, the proximal tibial epiphyseal ossification center appeared at the normal time (10–12 days). Soon thereafter, abnormalities were observed; in particular, spicules of osseous and marrow tissue connecting the epiphysis to the metaphysis through the epiphyseal plate were seen. By 21 to 48 days of age, all of 19 deficient offspring showed marked abnormalities. In over 30%, the epiphysis seemed absent when viewed from the external aspect; when examined at the internal (split) surface, a tiny epiphysis was found recessed into a pocket‐like depression in the metaphysis and partially fused with it. Even when “pocketing” was incomplete, the epiphysis was small, misshapen, poorly vascularized, fragmented, and showed rarefaction. Bony spicules frequently perforated the narrowed plate. Abnormalities of the cartilage were often seen. Offspring of deficient mothers transferred to the manganese‐supplemented diet late in gestation also showed some defects of this epiphysis. Possible mechanisms related to the pathogenesis of this disorder are discussed, and a hypothesi