Somatostatin has been implicated in the control of a number of hormonal systems, through its inhibitory effects on the release of somatotrophin, insulin, thyroid stimulating hormone and glucagon. It is therefore central to the control of metabolism and growth. There have been a number of trials based on neutralisation of the hormone by active immunisation against it, which have produced variable results. Trials in commercial breeds of sheep and cattle have generally resulted in increased growth rates, often associated with increased feed efficiency. The majority of these trials have been carried out on the males of the species; information on the female response is therefore limited. Work by Spencer et al (1985) on somatostatin immunisation in mixed groups of lambs did not result in significant increased in growth rate. However when the data was re-examined splitting the groups by sex, treated female lambs grew significantly better than their female control counterparts while there was no treatment response with male lambs. Recent work by Breier et a] (1988) has suggested that oestradiol 17-B, widely used as a growth promoter in ruminants, may affect the binding capacity of hepatic somatotrophic receptors thereby enhancing IGF-1 production and ultimately growth rate. This trial examines the effect of immunisation against somatostatin in pubertal heifers.