AbstractNOrmal and atypical follicular structures were classified and counted in the ovaries of guinea pigs 3 to 11 weeks of age. The per cent of primary and secondary follicles remain almost constant throughout the period of observation. Corpora lutea appeared at the fifth week and reached what appeared to be the mature level by the sixth week. The per cent of follicles having polynuclear ova reached an early peak at four weeks and decreased sharply at five weeks postpartum. The per cent of polyovular follicles increased to a low peak at nine weeks and decreased in frequency thereafter. The frequencies of atretic follicles with polynuclear ova and of quency thereafter. The frequencies of atretic follicles with polynuclear ova and of atretic polyovular follicles were significantly higher than that of atretic normal follicles. The results of this study were compared to simular observations on the Swiss mouse, Golden Hamster and Sprague‐Dawley rat. The data were discussed in relation to gonadotripic hormone level