Recent trials with beef-producing heifers have mostly been based on the use of an androgen, although oestrogens have been tested, alone or in combination. Thus, zeranol has been reported to increase gain (62, 63), while in other trials no response has been observed (37, 62). TBA administered alone (300 mg) has led to increases in weight gain and FCE of the order of 36% and 25% respectively (24, 27, 37, 64, 65, 66, 67, 68, 69, 70). In other trials, combinations of an oestrogen with TBA (68) or testosterone (62) have yielded significant growth responses. In general, it appears that the effect of TBA alone in heifers corresponds closely to the effect of combined oestrogen/TBA implants in steers.