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The widespread use of ultrasonography and IVF over the past decade has provided new tools to evaluate how follicles and oocytes react to different superstimulatory treatments. This information may be used to redefine actual hormonal stimulations to improve results of MOET programs and/or obtain improved responses from the "so-called" poor responders. This retrospective study examined data collected over a 5-year period involving oocyte collections in a commercial embryo transfer unit to determine the stimulation protocol that was most effective in producing competent cumulus oocyte complexes, and to determine a definition of a low responder. Overall, the population of small antral follicles at the time of follicle ablation was the most important factor affecting results. This pool of small antral follicles was significantly correlated with the number of follicles at oocyte collection, and to the number of viable and transferable embryos produced. Varying the superstimulatory treatments in terms of type of FSH in association with a shorter or longer coasting period did not affect ovarian response or embryonic development rates. Low responders (less than 10% of the animals in this study) were defined as animals with a lower than average follicular response following superstimulation. Low potential animals were defined as donors producing a limited number of embryos because of the limited population of small antral follicles present in the ovaries at initiation of FSH treatment. Embryo transfer practitioners must distinguish between low responders and low potential animals as modifications to the stimulation protocol for the latter group is unlikely to result in a higher number of transferable embryos.