Controlling the timing of ovulation in cows that fail to conceive after insemination would provide a further opportunity for scheduled rebreeding in a group of cows to save time and optimise results. The ear implant Crestar containing the synthetic progestagen norgestomet provides a means of achieving this control (Sinclair et al., 1992; Lowman et al., 1994). Removing the implant on day 25 after oestrus results in less false positive pregnancy diagnoses than removal on day 21 with the implant in place for nine/ten days. However, treatment of beef heifers with a norgestomet implant towards the end of luteal phase to control oestrus and ovulation is associated with development of a persistent ovulatory follicle and reduced pregnancy rate (Mihm et al., 1994). The objective of this study was to determine the effects of treating beef cows with one or two norgestomet implants on the development of the dominant ovulatory follicle of the second follicular wave.