It is generally believed that in adult rodents the vast majority of very small follicles (less than 20 granulosa cells in cross section) are not growing. However, the only data to support this assumption were collected with methods that may not be sufficiently sensitive to distinguish between nongrowing follicles and slowly growing follicles. In this study, the method of long-term continuous [3H]thymidine (3H-TdR) infusion was used to examine growth of very small follicles. Young adult rats were given continuous infusions of 3H-TdR for up to 7 days. Autoradiographs were prepared from 2-microns-thick sections of the ovaries. The proportion of labeled very small follicles increased as the duration of the infusion increased. After 7 days of continuous 3H-TdR infusion, 37% of follicles with 4 or fewer granulosa cells had at least one labeled granulosa cell, as did 68% of follicles with 5 8 granulosa cells, 86% of follicles with 9 12 granulosa cells, and 100% of follicles with 13 or more granulosa cells. The number of labeled cells per labeled follicle (the labeling index) also increased with increasing infusion time. However, few follicles had labeling indices of 100%, even after 7 days of continuous 3H-TdR infusion. These results suggest that a substantial fraction of very small follicles is growing in adult cycling rats; however, the rate of growth appears to be quite slow.