Recent advances in image technology, including significant gains in spatial resolution, have made realtime sequential ovarian evaluations possible in small rodents, allowing longitudinal (continued) studies of the ovarian cycle and reducing the required number of experimental animals. The aim of this study was to evaluate exogenous stimulated follicular growth in mice using high-resolution ultrasound technology. Female mice (n ¼ 15) received a 5 IU intraperitoneal injection of equine chorionic gonadotropin (eCG) and 48 h later a 5 IU injection of human chorionic gonadotropin (hCG), and were allowed to mate thereafter. In experiment 1, animals (n ¼ 7) were evaluated every 6 h, from 3 to 51 h after eCG injection, with an ultrasound biomicroscopy (UBM) equipped with a realtime 45 MHz microvisualization probe (RMV 707b). The ovaries were identified and follicular population quantified, and follicles were classified according to the diameter as small ( 449 mm) or large ( 450 mm). A significant change in the distribution of follicle population according to category was observed only 45 h after eCG injection (P, 0.05). In experiment 2, animals (n ¼ 8) were evaluated every 2 h, from 2 h to 10 h after hCG treatment. The largest follicles reached a maximum size (596.7+106.0 mm) 5.8+2.3 h after hCG injection. As expected, the population of large follicles decreased thereafter, indicating the progress of ovulations, but large follicles were still detected late after treatment (10.1+1.1 h). In conclusion, UBM can be used to evaluate follicle dynamics in superstimulated mice (C57BL/6 and BALB/c); significant changes in follicle distribution only occur at later stages after eCG stimulation; and hCG-induced ovulations may not occur synchronously in mice.