This study investigated the relationship between individual follicle growth, steroid release and follicular morphology to provide basic information about critical stages in follicle development. Preantral mouse follicles secreted significant and constant amounts of progesterone that were not related to oestradiol production but did appear to be related to thecal organization. Oestradiol release was variable among follicles of equivalent diameter, but marked increases in oestradiol concentrations were measured in follicles of 300 and 400 microns diameter. Over 4 days of culture, the proportion of follicles growing beyond the threshold diameter of 300 microns was lower in small (140-160 microns) than it was in large preantral follicles (170-210 microns). Retardation of growth below this diameter was associated with significantly decreased steroid concentrations. In follicles growing beyond the threshold diameter, antrum formation progressed, but full Graafian stage was not attained. Among large follicles, variability in oestradiol release could not be associated with obvious histological differences. In contrast, structural disturbances were associated with reduced steroid production and growth abnormalities. Therefore, it is concluded that the preantral follicles selected for culture comprise distinct follicle populations with respect to their developmental potential in vitro, and that follicle development is controlled by endogenous processes involving oestradiol at distinct stages of follicle development.