Small follicles, less than or equal to 100 microns, in monkey ovaries were divided into four types based on the morphological characteristics of the granulosa cells that surrounded the oocyte: primordial, intermediary, primary and secondary follicles. The proportion of primordial follicles positively correlated, whereas those of intermediary, primary and secondary follicles negatively correlated, with the total number of follicles less than or equal to 100 microns. There was no relationship between the population of nongrowing follicles (primordial and intermediary) and that of early-growing follicles (primary and secondary). Administration of exogenous gonadotrophins did not induce significant changes in the population of small follicles, whereas there was a significant increase in the number of intermediary follicles when gonadotrophins were associated with a gonadotrophin-releasing hormone agonist, buserelin. Buserelin can therefore partly inhibit the initiation of ovarian follicular growth in monkeys.