It was postulated that the induction of twin ovulations in cattle may require a combination of two appropriate stimuli. The first putative requirement is an increase in the circulating FSH concentrations and/or a decrease in inhibin concentrations to induce the development of additional potentially ovulatory follicles. The second putative requirement is an increase in LH pulse frequency during the follicular phase of the cycle to enhance the stimulus for follicle maturation and ovulation. On the basis of studies reported in the literature, it was suggested that such stimuli may be provided by immunization against inhibin and against a steroid hormone such as androstenedione, respectively. This experiment was designed to determine the effects of immunization against androstenedione and inhibin, either separately or together on the endocrine status of heifers and on the associated patterns of ovarian follicle development and ovulation rate.